Source code for torch.optim.lr_scheduler

import types
import math
from torch._six import inf
from functools import wraps
import warnings
import weakref
from collections import Counter
from bisect import bisect_right

from .optimizer import Optimizer


EPOCH_DEPRECATION_WARNING = (
    "The epoch parameter in `scheduler.step()` was not necessary and is being "
    "deprecated where possible. Please use `scheduler.step()` to step the "
    "scheduler. During the deprecation, if epoch is different from None, the "
    "closed form is used instead of the new chainable form, where available. "
    "Please open an issue if you are unable to replicate your use case: "
    "https://github.com/pytorch/pytorch/issues/new/choose."
)

class _LRScheduler(object):

    def __init__(self, optimizer, last_epoch=-1, verbose=False):

        # Attach optimizer
        if not isinstance(optimizer, Optimizer):
            raise TypeError('{} is not an Optimizer'.format(
                type(optimizer).__name__))
        self.optimizer = optimizer

        # Initialize epoch and base learning rates
        if last_epoch == -1:
            for group in optimizer.param_groups:
                group.setdefault('initial_lr', group['lr'])
        else:
            for i, group in enumerate(optimizer.param_groups):
                if 'initial_lr' not in group:
                    raise KeyError("param 'initial_lr' is not specified "
                                   "in param_groups[{}] when resuming an optimizer".format(i))
        self.base_lrs = [group['initial_lr'] for group in optimizer.param_groups]
        self.last_epoch = last_epoch

        # Following https://github.com/pytorch/pytorch/issues/20124
        # We would like to ensure that `lr_scheduler.step()` is called after
        # `optimizer.step()`
        def with_counter(method):
            if getattr(method, '_with_counter', False):
                # `optimizer.step()` has already been replaced, return.
                return method

            # Keep a weak reference to the optimizer instance to prevent
            # cyclic references.
            instance_ref = weakref.ref(method.__self__)
            # Get the unbound method for the same purpose.
            func = method.__func__
            cls = instance_ref().__class__
            del method

            @wraps(func)
            def wrapper(*args, **kwargs):
                instance = instance_ref()
                instance._step_count += 1
                wrapped = func.__get__(instance, cls)
                return wrapped(*args, **kwargs)

            # Note that the returned function here is no longer a bound method,
            # so attributes like `__func__` and `__self__` no longer exist.
            wrapper._with_counter = True
            return wrapper

        self.optimizer.step = with_counter(self.optimizer.step)
        self.optimizer._step_count = 0
        self._step_count = 0
        self.verbose = verbose

        self.step()

    def state_dict(self):
        """Returns the state of the scheduler as a :class:`dict`.

        It contains an entry for every variable in self.__dict__ which
        is not the optimizer.
        """
        return {key: value for key, value in self.__dict__.items() if key != 'optimizer'}

    def load_state_dict(self, state_dict):
        """Loads the schedulers state.

        Args:
            state_dict (dict): scheduler state. Should be an object returned
                from a call to :meth:`state_dict`.
        """
        self.__dict__.update(state_dict)

    def get_last_lr(self):
        """ Return last computed learning rate by current scheduler.
        """
        return self._last_lr

    def get_lr(self):
        # Compute learning rate using chainable form of the scheduler
        raise NotImplementedError

    def print_lr(self, is_verbose, group, lr, epoch=None):
        """Display the current learning rate.
        """
        if is_verbose:
            if epoch is None:
                print('Adjusting learning rate'
                      ' of group {} to {:.4e}.'.format(group, lr))
            else:
                print('Epoch {:5d}: adjusting learning rate'
                      ' of group {} to {:.4e}.'.format(epoch, group, lr))


    def step(self, epoch=None):
        # Raise a warning if old pattern is detected
        # https://github.com/pytorch/pytorch/issues/20124
        if self._step_count == 1:
            if not hasattr(self.optimizer.step, "_with_counter"):
                warnings.warn("Seems like `optimizer.step()` has been overridden after learning rate scheduler "
                              "initialization. Please, make sure to call `optimizer.step()` before "
                              "`lr_scheduler.step()`. See more details at "
                              "https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate", UserWarning)

            # Just check if there were two first lr_scheduler.step() calls before optimizer.step()
            elif self.optimizer._step_count < 1:
                warnings.warn("Detected call of `lr_scheduler.step()` before `optimizer.step()`. "
                              "In PyTorch 1.1.0 and later, you should call them in the opposite order: "
                              "`optimizer.step()` before `lr_scheduler.step()`.  Failure to do this "
                              "will result in PyTorch skipping the first value of the learning rate schedule. "
                              "See more details at "
                              "https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate", UserWarning)
        self._step_count += 1

        class _enable_get_lr_call:

            def __init__(self, o):
                self.o = o

            def __enter__(self):
                self.o._get_lr_called_within_step = True
                return self

            def __exit__(self, type, value, traceback):
                self.o._get_lr_called_within_step = False

        with _enable_get_lr_call(self):
            if epoch is None:
                self.last_epoch += 1
                values = self.get_lr()
            else:
                warnings.warn(EPOCH_DEPRECATION_WARNING, UserWarning)
                self.last_epoch = epoch
                if hasattr(self, "_get_closed_form_lr"):
                    values = self._get_closed_form_lr()
                else:
                    values = self.get_lr()

        for i, data in enumerate(zip(self.optimizer.param_groups, values)):
            param_group, lr = data
            param_group['lr'] = lr
            self.print_lr(self.verbose, i, lr, epoch)

        self._last_lr = [group['lr'] for group in self.optimizer.param_groups]


class LambdaLR(_LRScheduler):
    """Sets the learning rate of each parameter group to the initial lr
    times a given function. When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        lr_lambda (function or list): A function which computes a multiplicative
            factor given an integer parameter epoch, or a list of such
            functions, one for each group in optimizer.param_groups.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> # Assuming optimizer has two groups.
        >>> lambda1 = lambda epoch: epoch // 30
        >>> lambda2 = lambda epoch: 0.95 ** epoch
        >>> scheduler = LambdaLR(optimizer, lr_lambda=[lambda1, lambda2])
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, lr_lambda, last_epoch=-1, verbose=False):
        self.optimizer = optimizer

        if not isinstance(lr_lambda, list) and not isinstance(lr_lambda, tuple):
            self.lr_lambdas = [lr_lambda] * len(optimizer.param_groups)
        else:
            if len(lr_lambda) != len(optimizer.param_groups):
                raise ValueError("Expected {} lr_lambdas, but got {}".format(
                    len(optimizer.param_groups), len(lr_lambda)))
            self.lr_lambdas = list(lr_lambda)
        super(LambdaLR, self).__init__(optimizer, last_epoch, verbose)

    def state_dict(self):
        """Returns the state of the scheduler as a :class:`dict`.

        It contains an entry for every variable in self.__dict__ which
        is not the optimizer.
        The learning rate lambda functions will only be saved if they are callable objects
        and not if they are functions or lambdas.

        When saving or loading the scheduler, please make sure to also save or load the state of the optimizer.
        """

        state_dict = {key: value for key, value in self.__dict__.items() if key not in ('optimizer', 'lr_lambdas')}
        state_dict['lr_lambdas'] = [None] * len(self.lr_lambdas)

        for idx, fn in enumerate(self.lr_lambdas):
            if not isinstance(fn, types.FunctionType):
                state_dict['lr_lambdas'][idx] = fn.__dict__.copy()

        return state_dict

    def load_state_dict(self, state_dict):
        """Loads the schedulers state.

        When saving or loading the scheduler, please make sure to also save or load the state of the optimizer.

        Args:
            state_dict (dict): scheduler state. Should be an object returned
                from a call to :meth:`state_dict`.
        """

        lr_lambdas = state_dict.pop('lr_lambdas')
        self.__dict__.update(state_dict)
        # Restore state_dict keys in order to prevent side effects
        # https://github.com/pytorch/pytorch/issues/32756
        state_dict['lr_lambdas'] = lr_lambdas

        for idx, fn in enumerate(lr_lambdas):
            if fn is not None:
                self.lr_lambdas[idx].__dict__.update(fn)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.")

        return [base_lr * lmbda(self.last_epoch)
                for lmbda, base_lr in zip(self.lr_lambdas, self.base_lrs)]


class MultiplicativeLR(_LRScheduler):
    """Multiply the learning rate of each parameter group by the factor given
    in the specified function. When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        lr_lambda (function or list): A function which computes a multiplicative
            factor given an integer parameter epoch, or a list of such
            functions, one for each group in optimizer.param_groups.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> lmbda = lambda epoch: 0.95
        >>> scheduler = MultiplicativeLR(optimizer, lr_lambda=lmbda)
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, lr_lambda, last_epoch=-1, verbose=False):
        self.optimizer = optimizer

        if not isinstance(lr_lambda, list) and not isinstance(lr_lambda, tuple):
            self.lr_lambdas = [lr_lambda] * len(optimizer.param_groups)
        else:
            if len(lr_lambda) != len(optimizer.param_groups):
                raise ValueError("Expected {} lr_lambdas, but got {}".format(
                    len(optimizer.param_groups), len(lr_lambda)))
            self.lr_lambdas = list(lr_lambda)
        super(MultiplicativeLR, self).__init__(optimizer, last_epoch, verbose)

    def state_dict(self):
        """Returns the state of the scheduler as a :class:`dict`.

        It contains an entry for every variable in self.__dict__ which
        is not the optimizer.
        The learning rate lambda functions will only be saved if they are callable objects
        and not if they are functions or lambdas.
        """
        state_dict = {key: value for key, value in self.__dict__.items() if key not in ('optimizer', 'lr_lambdas')}
        state_dict['lr_lambdas'] = [None] * len(self.lr_lambdas)

        for idx, fn in enumerate(self.lr_lambdas):
            if not isinstance(fn, types.FunctionType):
                state_dict['lr_lambdas'][idx] = fn.__dict__.copy()

        return state_dict

    def load_state_dict(self, state_dict):
        """Loads the schedulers state.

        Args:
            state_dict (dict): scheduler state. Should be an object returned
                from a call to :meth:`state_dict`.
        """
        lr_lambdas = state_dict.pop('lr_lambdas')
        self.__dict__.update(state_dict)
        # Restore state_dict keys in order to prevent side effects
        # https://github.com/pytorch/pytorch/issues/32756
        state_dict['lr_lambdas'] = lr_lambdas

        for idx, fn in enumerate(lr_lambdas):
            if fn is not None:
                self.lr_lambdas[idx].__dict__.update(fn)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch > 0:
            return [group['lr'] * lmbda(self.last_epoch)
                    for lmbda, group in zip(self.lr_lambdas, self.optimizer.param_groups)]
        else:
            return [group['lr'] for group in self.optimizer.param_groups]


class StepLR(_LRScheduler):
    """Decays the learning rate of each parameter group by gamma every
    step_size epochs. Notice that such decay can happen simultaneously with
    other changes to the learning rate from outside this scheduler. When
    last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        step_size (int): Period of learning rate decay.
        gamma (float): Multiplicative factor of learning rate decay.
            Default: 0.1.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> # Assuming optimizer uses lr = 0.05 for all groups
        >>> # lr = 0.05     if epoch < 30
        >>> # lr = 0.005    if 30 <= epoch < 60
        >>> # lr = 0.0005   if 60 <= epoch < 90
        >>> # ...
        >>> scheduler = StepLR(optimizer, step_size=30, gamma=0.1)
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, step_size, gamma=0.1, last_epoch=-1, verbose=False):
        self.step_size = step_size
        self.gamma = gamma
        super(StepLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if (self.last_epoch == 0) or (self.last_epoch % self.step_size != 0):
            return [group['lr'] for group in self.optimizer.param_groups]
        return [group['lr'] * self.gamma
                for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        return [base_lr * self.gamma ** (self.last_epoch // self.step_size)
                for base_lr in self.base_lrs]


class MultiStepLR(_LRScheduler):
    """Decays the learning rate of each parameter group by gamma once the
    number of epoch reaches one of the milestones. Notice that such decay can
    happen simultaneously with other changes to the learning rate from outside
    this scheduler. When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        milestones (list): List of epoch indices. Must be increasing.
        gamma (float): Multiplicative factor of learning rate decay.
            Default: 0.1.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> # Assuming optimizer uses lr = 0.05 for all groups
        >>> # lr = 0.05     if epoch < 30
        >>> # lr = 0.005    if 30 <= epoch < 80
        >>> # lr = 0.0005   if epoch >= 80
        >>> scheduler = MultiStepLR(optimizer, milestones=[30,80], gamma=0.1)
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, milestones, gamma=0.1, last_epoch=-1, verbose=False):
        self.milestones = Counter(milestones)
        self.gamma = gamma
        super(MultiStepLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch not in self.milestones:
            return [group['lr'] for group in self.optimizer.param_groups]
        return [group['lr'] * self.gamma ** self.milestones[self.last_epoch]
                for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        milestones = list(sorted(self.milestones.elements()))
        return [base_lr * self.gamma ** bisect_right(milestones, self.last_epoch)
                for base_lr in self.base_lrs]


class ConstantLR(_LRScheduler):
    """Decays the learning rate of each parameter group by a small constant factor until the
    number of epoch reaches a pre-defined milestone: total_iters. Notice that such decay can
    happen simultaneously with other changes to the learning rate from outside this scheduler.
    When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        factor (float): The number we multiply learning rate until the milestone. Default: 1./3.
        total_iters (int): The number of steps that the scheduler decays the learning rate.
            Default: 5.
        last_epoch (int): The index of the last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> # Assuming optimizer uses lr = 0.05 for all groups
        >>> # lr = 0.025   if epoch == 0
        >>> # lr = 0.025   if epoch == 1
        >>> # lr = 0.025   if epoch == 2
        >>> # lr = 0.025   if epoch == 3
        >>> # lr = 0.05    if epoch >= 4
        >>> scheduler = ConstantLR(self.opt, factor=0.5, total_iters=4)
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, factor=1.0 / 3, total_iters=5, last_epoch=-1, verbose=False):
        if factor > 1.0 or factor < 0:
            raise ValueError('Constant multiplicative factor expected to be between 0 and 1.')

        self.factor = factor
        self.total_iters = total_iters
        super(ConstantLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch == 0:
            return [group['lr'] * self.factor for group in self.optimizer.param_groups]

        if (self.last_epoch > self.total_iters or
                (self.last_epoch != self.total_iters)):
            return [group['lr'] for group in self.optimizer.param_groups]

        if (self.last_epoch == self.total_iters):
            return [group['lr'] * (1.0 / self.factor) for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        return [base_lr * (self.factor + (self.last_epoch >= self.total_iters) * (1 - self.factor))
                for base_lr in self.base_lrs]


class LinearLR(_LRScheduler):
    """Decays the learning rate of each parameter group by linearly changing small
    multiplicative factor until the number of epoch reaches a pre-defined milestone: total_iters.
    Notice that such decay can happen simultaneously with other changes to the learning rate
    from outside this scheduler. When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        start_factor (float): The number we multiply learning rate in the first epoch.
            The multiplication factor changes towards end_factor in the following epochs.
            Default: 1./3.
        end_factor (float): The number we multiply learning rate at the end of linear changing
            process. Default: 1.0.
        total_iters (int): The number of iterations that multiplicative factor reaches to 1.
            Default: 5.
        last_epoch (int): The index of the last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> # Assuming optimizer uses lr = 0.05 for all groups
        >>> # lr = 0.025    if epoch == 0
        >>> # lr = 0.03125  if epoch == 1
        >>> # lr = 0.0375   if epoch == 2
        >>> # lr = 0.04375  if epoch == 3
        >>> # lr = 0.005    if epoch >= 4
        >>> scheduler = LinearLR(self.opt, start_factor=0.5, total_iters=4)
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, start_factor=1.0 / 3, end_factor=1.0, total_iters=5, last_epoch=-1,
                 verbose=False):
        if start_factor > 1.0 or start_factor < 0:
            raise ValueError('Starting multiplicative factor expected to be between 0 and 1.')

        if end_factor > 1.0 or end_factor < 0:
            raise ValueError('Ending multiplicative factor expected to be between 0 and 1.')

        self.start_factor = start_factor
        self.end_factor = end_factor
        self.total_iters = total_iters
        super(LinearLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch == 0:
            return [group['lr'] * self.start_factor for group in self.optimizer.param_groups]

        if (self.last_epoch > self.total_iters):
            return [group['lr'] for group in self.optimizer.param_groups]

        return [group['lr'] * (1. + (self.end_factor - self.start_factor) /
                (self.total_iters * self.start_factor + (self.last_epoch - 1) * (self.end_factor - self.start_factor)))
                for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        return [base_lr * (self.start_factor +
                (self.end_factor - self.start_factor) * min(self.total_iters, self.last_epoch) / self.total_iters)
                for base_lr in self.base_lrs]


class ExponentialLR(_LRScheduler):
    """Decays the learning rate of each parameter group by gamma every epoch.
    When last_epoch=-1, sets initial lr as lr.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        gamma (float): Multiplicative factor of learning rate decay.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.
    """

    def __init__(self, optimizer, gamma, last_epoch=-1, verbose=False):
        self.gamma = gamma
        super(ExponentialLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch == 0:
            return [group['lr'] for group in self.optimizer.param_groups]
        return [group['lr'] * self.gamma
                for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        return [base_lr * self.gamma ** self.last_epoch
                for base_lr in self.base_lrs]


class SequentialLR(_LRScheduler):
    """Receives the list of schedulers that is expected to be called sequentially during
    optimization process and milestone points that provides exact intervals to reflect
    which scheduler is supposed to be called at a given epoch.

    Args:
        schedulers (list): List of chained schedulers.
        milestones (list): List of integers that reflects milestone points.

    Example:
        >>> # Assuming optimizer uses lr = 1. for all groups
        >>> # lr = 0.1     if epoch == 0
        >>> # lr = 0.1     if epoch == 1
        >>> # lr = 0.9     if epoch == 2
        >>> # lr = 0.81    if epoch == 3
        >>> # lr = 0.729   if epoch == 4
        >>> scheduler1 = ConstantLR(self.opt, factor=0.1, total_iters=2)
        >>> scheduler2 = ExponentialLR(self.opt, gamma=0.9)
        >>> scheduler = SequentialLR(self.opt, schedulers=[scheduler1, scheduler2], milestones=[2])
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, optimizer, schedulers, milestones, last_epoch=-1, verbose=False):
        for scheduler_idx in range(1, len(schedulers)):
            if (schedulers[scheduler_idx].optimizer != schedulers[0].optimizer):
                raise ValueError(
                    "Sequential Schedulers expects all schedulers to belong to the same optimizer, but "
                    "got schedulers at index {} and {} to be different".format(0, scheduler_idx)
                )
        if (len(milestones) != len(schedulers) - 1):
            raise ValueError(
                "Sequential Schedulers expects number of schedulers provided to be one more "
                "than the number of milestone points, but got number of schedulers {} and the "
                "number of milestones to be equal to {}".format(len(schedulers), len(milestones))
            )
        self._schedulers = schedulers
        self._milestones = milestones
        self.last_epoch = last_epoch + 1

    def step(self):
        self.last_epoch += 1
        idx = bisect_right(self._milestones, self.last_epoch)
        if idx > 0 and self._milestones[idx - 1] == self.last_epoch:
            self._schedulers[idx].step(0)
        else:
            self._schedulers[idx].step()

    def state_dict(self):
        """Returns the state of the scheduler as a :class:`dict`.

        It contains an entry for every variable in self.__dict__ which
        is not the optimizer.
        The wrapped scheduler states will also be saved.
        """
        state_dict = {key: value for key, value in self.__dict__.items() if key not in ('optimizer', '_schedulers')}
        state_dict['_schedulers'] = [None] * len(self._schedulers)

        for idx, s in enumerate(self._schedulers):
            state_dict['_schedulers'][idx] = s.state_dict()

        return state_dict

    def load_state_dict(self, state_dict):
        """Loads the schedulers state.

        Args:
            state_dict (dict): scheduler state. Should be an object returned
                from a call to :meth:`state_dict`.
        """
        _schedulers = state_dict.pop('_schedulers')
        self.__dict__.update(state_dict)
        # Restore state_dict keys in order to prevent side effects
        # https://github.com/pytorch/pytorch/issues/32756
        state_dict['_schedulers'] = _schedulers

        for idx, s in enumerate(_schedulers):
            self._schedulers[idx].load_state_dict(s)


class CosineAnnealingLR(_LRScheduler):
    r"""Set the learning rate of each parameter group using a cosine annealing
    schedule, where :math:`\eta_{max}` is set to the initial lr and
    :math:`T_{cur}` is the number of epochs since the last restart in SGDR:

    .. math::
        \begin{aligned}
            \eta_t & = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1
            + \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right),
            & T_{cur} \neq (2k+1)T_{max}; \\
            \eta_{t+1} & = \eta_{t} + \frac{1}{2}(\eta_{max} - \eta_{min})
            \left(1 - \cos\left(\frac{1}{T_{max}}\pi\right)\right),
            & T_{cur} = (2k+1)T_{max}.
        \end{aligned}

    When last_epoch=-1, sets initial lr as lr. Notice that because the schedule
    is defined recursively, the learning rate can be simultaneously modified
    outside this scheduler by other operators. If the learning rate is set
    solely by this scheduler, the learning rate at each step becomes:

    .. math::
        \eta_t = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1 +
        \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right)

    It has been proposed in
    `SGDR: Stochastic Gradient Descent with Warm Restarts`_. Note that this only
    implements the cosine annealing part of SGDR, and not the restarts.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        T_max (int): Maximum number of iterations.
        eta_min (float): Minimum learning rate. Default: 0.
        last_epoch (int): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    .. _SGDR\: Stochastic Gradient Descent with Warm Restarts:
        https://arxiv.org/abs/1608.03983
    """

    def __init__(self, optimizer, T_max, eta_min=0, last_epoch=-1, verbose=False):
        self.T_max = T_max
        self.eta_min = eta_min
        super(CosineAnnealingLR, self).__init__(optimizer, last_epoch, verbose)

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        if self.last_epoch == 0:
            return [group['lr'] for group in self.optimizer.param_groups]
        elif (self.last_epoch - 1 - self.T_max) % (2 * self.T_max) == 0:
            return [group['lr'] + (base_lr - self.eta_min) *
                    (1 - math.cos(math.pi / self.T_max)) / 2
                    for base_lr, group in
                    zip(self.base_lrs, self.optimizer.param_groups)]
        return [(1 + math.cos(math.pi * self.last_epoch / self.T_max)) /
                (1 + math.cos(math.pi * (self.last_epoch - 1) / self.T_max)) *
                (group['lr'] - self.eta_min) + self.eta_min
                for group in self.optimizer.param_groups]

    def _get_closed_form_lr(self):
        return [self.eta_min + (base_lr - self.eta_min) *
                (1 + math.cos(math.pi * self.last_epoch / self.T_max)) / 2
                for base_lr in self.base_lrs]


class ChainedScheduler(_LRScheduler):
    """Chains list of learning rate schedulers. It takes a list of chainable learning
    rate schedulers and performs consecutive step() functions belong to them by just
    one call.

    Args:
        schedulers (list): List of chained schedulers.

    Example:
        >>> # Assuming optimizer uses lr = 1. for all groups
        >>> # lr = 0.09     if epoch == 0
        >>> # lr = 0.081    if epoch == 1
        >>> # lr = 0.729    if epoch == 2
        >>> # lr = 0.6561   if epoch == 3
        >>> # lr = 0.59049  if epoch >= 4
        >>> scheduler1 = ConstantLR(self.opt, factor=0.1, total_iters=2)
        >>> scheduler2 = ExponentialLR(self.opt, gamma=0.9)
        >>> scheduler = ChainedScheduler([scheduler1, scheduler2])
        >>> for epoch in range(100):
        >>>     train(...)
        >>>     validate(...)
        >>>     scheduler.step()
    """

    def __init__(self, schedulers):
        for scheduler_idx in range(1, len(schedulers)):
            if (schedulers[scheduler_idx].optimizer != schedulers[0].optimizer):
                raise ValueError(
                    "ChainedScheduler expects all schedulers to belong to the same optimizer, but "
                    "got schedulers at index {} and {} to be different".format(0, scheduler_idx)
                )
        self._schedulers = list(schedulers)

    def step(self):
        for scheduler in self._schedulers:
            scheduler.step()

    def state_dict(self):
        """Returns the state of the scheduler as a :class:`dict`.

        It contains an entry for every variable in self.__dict__ which
        is not the optimizer.
        The wrapped scheduler states will also be saved.
        """
        state_dict = {key: value for key, value in self.__dict__.items() if key not in ('optimizer', '_schedulers')}
        state_dict['_schedulers'] = [None] * len(self._schedulers)

        for idx, s in enumerate(self._schedulers):
            state_dict['_schedulers'][idx] = s.state_dict()

        return state_dict

    def load_state_dict(self, state_dict):
        """Loads the schedulers state.

        Args:
            state_dict (dict): scheduler state. Should be an object returned
                from a call to :meth:`state_dict`.
        """
        _schedulers = state_dict.pop('_schedulers')
        self.__dict__.update(state_dict)
        # Restore state_dict keys in order to prevent side effects
        # https://github.com/pytorch/pytorch/issues/32756
        state_dict['_schedulers'] = _schedulers

        for idx, s in enumerate(_schedulers):
            self._schedulers[idx].load_state_dict(s)


class ReduceLROnPlateau(object):
    """Reduce learning rate when a metric has stopped improving.
    Models often benefit from reducing the learning rate by a factor
    of 2-10 once learning stagnates. This scheduler reads a metrics
    quantity and if no improvement is seen for a 'patience' number
    of epochs, the learning rate is reduced.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        mode (str): One of `min`, `max`. In `min` mode, lr will
            be reduced when the quantity monitored has stopped
            decreasing; in `max` mode it will be reduced when the
            quantity monitored has stopped increasing. Default: 'min'.
        factor (float): Factor by which the learning rate will be
            reduced. new_lr = lr * factor. Default: 0.1.
        patience (int): Number of epochs with no improvement after
            which learning rate will be reduced. For example, if
            `patience = 2`, then we will ignore the first 2 epochs
            with no improvement, and will only decrease the LR after the
            3rd epoch if the loss still hasn't improved then.
            Default: 10.
        threshold (float): Threshold for measuring the new optimum,
            to only focus on significant changes. Default: 1e-4.
        threshold_mode (str): One of `rel`, `abs`. In `rel` mode,
            dynamic_threshold = best * ( 1 + threshold ) in 'max'
            mode or best * ( 1 - threshold ) in `min` mode.
            In `abs` mode, dynamic_threshold = best + threshold in
            `max` mode or best - threshold in `min` mode. Default: 'rel'.
        cooldown (int): Number of epochs to wait before resuming
            normal operation after lr has been reduced. Default: 0.
        min_lr (float or list): A scalar or a list of scalars. A
            lower bound on the learning rate of all param groups
            or each group respectively. Default: 0.
        eps (float): Minimal decay applied to lr. If the difference
            between new and old lr is smaller than eps, the update is
            ignored. Default: 1e-8.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> optimizer = torch.optim.SGD(model.parameters(), lr=0.1, momentum=0.9)
        >>> scheduler = ReduceLROnPlateau(optimizer, 'min')
        >>> for epoch in range(10):
        >>>     train(...)
        >>>     val_loss = validate(...)
        >>>     # Note that step should be called after validate()
        >>>     scheduler.step(val_loss)
    """

    def __init__(self, optimizer, mode='min', factor=0.1, patience=10,
                 threshold=1e-4, threshold_mode='rel', cooldown=0,
                 min_lr=0, eps=1e-8, verbose=False):

        if factor >= 1.0:
            raise ValueError('Factor should be < 1.0.')
        self.factor = factor

        # Attach optimizer
        if not isinstance(optimizer, Optimizer):
            raise TypeError('{} is not an Optimizer'.format(
                type(optimizer).__name__))
        self.optimizer = optimizer

        if isinstance(min_lr, list) or isinstance(min_lr, tuple):
            if len(min_lr) != len(optimizer.param_groups):
                raise ValueError("expected {} min_lrs, got {}".format(
                    len(optimizer.param_groups), len(min_lr)))
            self.min_lrs = list(min_lr)
        else:
            self.min_lrs = [min_lr] * len(optimizer.param_groups)

        self.patience = patience
        self.verbose = verbose
        self.cooldown = cooldown
        self.cooldown_counter = 0
        self.mode = mode
        self.threshold = threshold
        self.threshold_mode = threshold_mode
        self.best = None
        self.num_bad_epochs = None
        self.mode_worse = None  # the worse value for the chosen mode
        self.eps = eps
        self.last_epoch = 0
        self._init_is_better(mode=mode, threshold=threshold,
                             threshold_mode=threshold_mode)
        self._reset()

    def _reset(self):
        """Resets num_bad_epochs counter and cooldown counter."""
        self.best = self.mode_worse
        self.cooldown_counter = 0
        self.num_bad_epochs = 0

    def step(self, metrics, epoch=None):
        # convert `metrics` to float, in case it's a zero-dim Tensor
        current = float(metrics)
        if epoch is None:
            epoch = self.last_epoch + 1
        else:
            warnings.warn(EPOCH_DEPRECATION_WARNING, UserWarning)
        self.last_epoch = epoch

        if self.is_better(current, self.best):
            self.best = current
            self.num_bad_epochs = 0
        else:
            self.num_bad_epochs += 1

        if self.in_cooldown:
            self.cooldown_counter -= 1
            self.num_bad_epochs = 0  # ignore any bad epochs in cooldown

        if self.num_bad_epochs > self.patience:
            self._reduce_lr(epoch)
            self.cooldown_counter = self.cooldown
            self.num_bad_epochs = 0

        self._last_lr = [group['lr'] for group in self.optimizer.param_groups]

    def _reduce_lr(self, epoch):
        for i, param_group in enumerate(self.optimizer.param_groups):
            old_lr = float(param_group['lr'])
            new_lr = max(old_lr * self.factor, self.min_lrs[i])
            if old_lr - new_lr > self.eps:
                param_group['lr'] = new_lr
                if self.verbose:
                    print('Epoch {:5d}: reducing learning rate'
                          ' of group {} to {:.4e}.'.format(epoch, i, new_lr))

    @property
    def in_cooldown(self):
        return self.cooldown_counter > 0

    def is_better(self, a, best):
        if self.mode == 'min' and self.threshold_mode == 'rel':
            rel_epsilon = 1. - self.threshold
            return a < best * rel_epsilon

        elif self.mode == 'min' and self.threshold_mode == 'abs':
            return a < best - self.threshold

        elif self.mode == 'max' and self.threshold_mode == 'rel':
            rel_epsilon = self.threshold + 1.
            return a > best * rel_epsilon

        else:  # mode == 'max' and epsilon_mode == 'abs':
            return a > best + self.threshold

    def _init_is_better(self, mode, threshold, threshold_mode):
        if mode not in {'min', 'max'}:
            raise ValueError('mode ' + mode + ' is unknown!')
        if threshold_mode not in {'rel', 'abs'}:
            raise ValueError('threshold mode ' + threshold_mode + ' is unknown!')

        if mode == 'min':
            self.mode_worse = inf
        else:  # mode == 'max':
            self.mode_worse = -inf

        self.mode = mode
        self.threshold = threshold
        self.threshold_mode = threshold_mode

    def state_dict(self):
        return {key: value for key, value in self.__dict__.items() if key != 'optimizer'}

    def load_state_dict(self, state_dict):
        self.__dict__.update(state_dict)
        self._init_is_better(mode=self.mode, threshold=self.threshold, threshold_mode=self.threshold_mode)


class CyclicLR(_LRScheduler):
    r"""Sets the learning rate of each parameter group according to
    cyclical learning rate policy (CLR). The policy cycles the learning
    rate between two boundaries with a constant frequency, as detailed in
    the paper `Cyclical Learning Rates for Training Neural Networks`_.
    The distance between the two boundaries can be scaled on a per-iteration
    or per-cycle basis.

    Cyclical learning rate policy changes the learning rate after every batch.
    `step` should be called after a batch has been used for training.

    This class has three built-in policies, as put forth in the paper:

    * "triangular": A basic triangular cycle without amplitude scaling.
    * "triangular2": A basic triangular cycle that scales initial amplitude by half each cycle.
    * "exp_range": A cycle that scales initial amplitude by :math:`\text{gamma}^{\text{cycle iterations}}`
      at each cycle iteration.

    This implementation was adapted from the github repo: `bckenstler/CLR`_

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        base_lr (float or list): Initial learning rate which is the
            lower boundary in the cycle for each parameter group.
        max_lr (float or list): Upper learning rate boundaries in the cycle
            for each parameter group. Functionally,
            it defines the cycle amplitude (max_lr - base_lr).
            The lr at any cycle is the sum of base_lr
            and some scaling of the amplitude; therefore
            max_lr may not actually be reached depending on
            scaling function.
        step_size_up (int): Number of training iterations in the
            increasing half of a cycle. Default: 2000
        step_size_down (int): Number of training iterations in the
            decreasing half of a cycle. If step_size_down is None,
            it is set to step_size_up. Default: None
        mode (str): One of {triangular, triangular2, exp_range}.
            Values correspond to policies detailed above.
            If scale_fn is not None, this argument is ignored.
            Default: 'triangular'
        gamma (float): Constant in 'exp_range' scaling function:
            gamma**(cycle iterations)
            Default: 1.0
        scale_fn (function): Custom scaling policy defined by a single
            argument lambda function, where
            0 <= scale_fn(x) <= 1 for all x >= 0.
            If specified, then 'mode' is ignored.
            Default: None
        scale_mode (str): {'cycle', 'iterations'}.
            Defines whether scale_fn is evaluated on
            cycle number or cycle iterations (training
            iterations since start of cycle).
            Default: 'cycle'
        cycle_momentum (bool): If ``True``, momentum is cycled inversely
            to learning rate between 'base_momentum' and 'max_momentum'.
            Default: True
        base_momentum (float or list): Lower momentum boundaries in the cycle
            for each parameter group. Note that momentum is cycled inversely
            to learning rate; at the peak of a cycle, momentum is
            'base_momentum' and learning rate is 'max_lr'.
            Default: 0.8
        max_momentum (float or list): Upper momentum boundaries in the cycle
            for each parameter group. Functionally,
            it defines the cycle amplitude (max_momentum - base_momentum).
            The momentum at any cycle is the difference of max_momentum
            and some scaling of the amplitude; therefore
            base_momentum may not actually be reached depending on
            scaling function. Note that momentum is cycled inversely
            to learning rate; at the start of a cycle, momentum is 'max_momentum'
            and learning rate is 'base_lr'
            Default: 0.9
        last_epoch (int): The index of the last batch. This parameter is used when
            resuming a training job. Since `step()` should be invoked after each
            batch instead of after each epoch, this number represents the total
            number of *batches* computed, not the total number of epochs computed.
            When last_epoch=-1, the schedule is started from the beginning.
            Default: -1
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> optimizer = torch.optim.SGD(model.parameters(), lr=0.1, momentum=0.9)
        >>> scheduler = torch.optim.lr_scheduler.CyclicLR(optimizer, base_lr=0.01, max_lr=0.1)
        >>> data_loader = torch.utils.data.DataLoader(...)
        >>> for epoch in range(10):
        >>>     for batch in data_loader:
        >>>         train_batch(...)
        >>>         scheduler.step()


    .. _Cyclical Learning Rates for Training Neural Networks: https://arxiv.org/abs/1506.01186
    .. _bckenstler/CLR: https://github.com/bckenstler/CLR
    """

    def __init__(self,
                 optimizer,
                 base_lr,
                 max_lr,
                 step_size_up=2000,
                 step_size_down=None,
                 mode='triangular',
                 gamma=1.,
                 scale_fn=None,
                 scale_mode='cycle',
                 cycle_momentum=True,
                 base_momentum=0.8,
                 max_momentum=0.9,
                 last_epoch=-1,
                 verbose=False):

        # Attach optimizer
        if not isinstance(optimizer, Optimizer):
            raise TypeError('{} is not an Optimizer'.format(
                type(optimizer).__name__))
        self.optimizer = optimizer

        base_lrs = self._format_param('base_lr', optimizer, base_lr)
        if last_epoch == -1:
            for lr, group in zip(base_lrs, optimizer.param_groups):
                group['lr'] = lr

        self.max_lrs = self._format_param('max_lr', optimizer, max_lr)

        step_size_up = float(step_size_up)
        step_size_down = float(step_size_down) if step_size_down is not None else step_size_up
        self.total_size = step_size_up + step_size_down
        self.step_ratio = step_size_up / self.total_size

        if mode not in ['triangular', 'triangular2', 'exp_range'] \
                and scale_fn is None:
            raise ValueError('mode is invalid and scale_fn is None')

        self.mode = mode
        self.gamma = gamma

        if scale_fn is None:
            if self.mode == 'triangular':
                self.scale_fn = self._triangular_scale_fn
                self.scale_mode = 'cycle'
            elif self.mode == 'triangular2':
                self.scale_fn = self._triangular2_scale_fn
                self.scale_mode = 'cycle'
            elif self.mode == 'exp_range':
                self.scale_fn = self._exp_range_scale_fn
                self.scale_mode = 'iterations'
        else:
            self.scale_fn = scale_fn
            self.scale_mode = scale_mode

        self.cycle_momentum = cycle_momentum
        if cycle_momentum:
            if 'momentum' not in optimizer.defaults:
                raise ValueError('optimizer must support momentum with `cycle_momentum` option enabled')

            base_momentums = self._format_param('base_momentum', optimizer, base_momentum)
            if last_epoch == -1:
                for momentum, group in zip(base_momentums, optimizer.param_groups):
                    group['momentum'] = momentum
            self.base_momentums = [group['momentum'] for group in optimizer.param_groups]
            self.max_momentums = self._format_param('max_momentum', optimizer, max_momentum)

        super(CyclicLR, self).__init__(optimizer, last_epoch, verbose)
        self.base_lrs = base_lrs

    def _format_param(self, name, optimizer, param):
        """Return correctly formatted lr/momentum for each param group."""
        if isinstance(param, (list, tuple)):
            if len(param) != len(optimizer.param_groups):
                raise ValueError("expected {} values for {}, got {}".format(
                    len(optimizer.param_groups), name, len(param)))
            return param
        else:
            return [param] * len(optimizer.param_groups)

    def _triangular_scale_fn(self, x):
        return 1.

    def _triangular2_scale_fn(self, x):
        return 1 / (2. ** (x - 1))

    def _exp_range_scale_fn(self, x):
        return self.gamma**(x)

    def get_lr(self):
        """Calculates the learning rate at batch index. This function treats
        `self.last_epoch` as the last batch index.

        If `self.cycle_momentum` is ``True``, this function has a side effect of
        updating the optimizer's momentum.
        """

        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        cycle = math.floor(1 + self.last_epoch / self.total_size)
        x = 1. + self.last_epoch / self.total_size - cycle
        if x <= self.step_ratio:
            scale_factor = x / self.step_ratio
        else:
            scale_factor = (x - 1) / (self.step_ratio - 1)

        lrs = []
        for base_lr, max_lr in zip(self.base_lrs, self.max_lrs):
            base_height = (max_lr - base_lr) * scale_factor
            if self.scale_mode == 'cycle':
                lr = base_lr + base_height * self.scale_fn(cycle)
            else:
                lr = base_lr + base_height * self.scale_fn(self.last_epoch)
            lrs.append(lr)

        if self.cycle_momentum:
            momentums = []
            for base_momentum, max_momentum in zip(self.base_momentums, self.max_momentums):
                base_height = (max_momentum - base_momentum) * scale_factor
                if self.scale_mode == 'cycle':
                    momentum = max_momentum - base_height * self.scale_fn(cycle)
                else:
                    momentum = max_momentum - base_height * self.scale_fn(self.last_epoch)
                momentums.append(momentum)
            for param_group, momentum in zip(self.optimizer.param_groups, momentums):
                param_group['momentum'] = momentum

        return lrs


class CosineAnnealingWarmRestarts(_LRScheduler):
    r"""Set the learning rate of each parameter group using a cosine annealing
    schedule, where :math:`\eta_{max}` is set to the initial lr, :math:`T_{cur}`
    is the number of epochs since the last restart and :math:`T_{i}` is the number
    of epochs between two warm restarts in SGDR:

    .. math::
        \eta_t = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1 +
        \cos\left(\frac{T_{cur}}{T_{i}}\pi\right)\right)

    When :math:`T_{cur}=T_{i}`, set :math:`\eta_t = \eta_{min}`.
    When :math:`T_{cur}=0` after restart, set :math:`\eta_t=\eta_{max}`.

    It has been proposed in
    `SGDR: Stochastic Gradient Descent with Warm Restarts`_.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        T_0 (int): Number of iterations for the first restart.
        T_mult (int, optional): A factor increases :math:`T_{i}` after a restart. Default: 1.
        eta_min (float, optional): Minimum learning rate. Default: 0.
        last_epoch (int, optional): The index of last epoch. Default: -1.
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    .. _SGDR\: Stochastic Gradient Descent with Warm Restarts:
        https://arxiv.org/abs/1608.03983
    """

    def __init__(self, optimizer, T_0, T_mult=1, eta_min=0, last_epoch=-1, verbose=False):
        if T_0 <= 0 or not isinstance(T_0, int):
            raise ValueError("Expected positive integer T_0, but got {}".format(T_0))
        if T_mult < 1 or not isinstance(T_mult, int):
            raise ValueError("Expected integer T_mult >= 1, but got {}".format(T_mult))
        self.T_0 = T_0
        self.T_i = T_0
        self.T_mult = T_mult
        self.eta_min = eta_min

        super(CosineAnnealingWarmRestarts, self).__init__(optimizer, last_epoch, verbose)

        self.T_cur = self.last_epoch

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        return [self.eta_min + (base_lr - self.eta_min) * (1 + math.cos(math.pi * self.T_cur / self.T_i)) / 2
                for base_lr in self.base_lrs]

    def step(self, epoch=None):
        """Step could be called after every batch update

        Example:
            >>> scheduler = CosineAnnealingWarmRestarts(optimizer, T_0, T_mult)
            >>> iters = len(dataloader)
            >>> for epoch in range(20):
            >>>     for i, sample in enumerate(dataloader):
            >>>         inputs, labels = sample['inputs'], sample['labels']
            >>>         optimizer.zero_grad()
            >>>         outputs = net(inputs)
            >>>         loss = criterion(outputs, labels)
            >>>         loss.backward()
            >>>         optimizer.step()
            >>>         scheduler.step(epoch + i / iters)

        This function can be called in an interleaved way.

        Example:
            >>> scheduler = CosineAnnealingWarmRestarts(optimizer, T_0, T_mult)
            >>> for epoch in range(20):
            >>>     scheduler.step()
            >>> scheduler.step(26)
            >>> scheduler.step() # scheduler.step(27), instead of scheduler(20)
        """

        if epoch is None and self.last_epoch < 0:
            epoch = 0

        if epoch is None:
            epoch = self.last_epoch + 1
            self.T_cur = self.T_cur + 1
            if self.T_cur >= self.T_i:
                self.T_cur = self.T_cur - self.T_i
                self.T_i = self.T_i * self.T_mult
        else:
            if epoch < 0:
                raise ValueError("Expected non-negative epoch, but got {}".format(epoch))
            if epoch >= self.T_0:
                if self.T_mult == 1:
                    self.T_cur = epoch % self.T_0
                else:
                    n = int(math.log((epoch / self.T_0 * (self.T_mult - 1) + 1), self.T_mult))
                    self.T_cur = epoch - self.T_0 * (self.T_mult ** n - 1) / (self.T_mult - 1)
                    self.T_i = self.T_0 * self.T_mult ** (n)
            else:
                self.T_i = self.T_0
                self.T_cur = epoch
        self.last_epoch = math.floor(epoch)

        class _enable_get_lr_call:

            def __init__(self, o):
                self.o = o

            def __enter__(self):
                self.o._get_lr_called_within_step = True
                return self

            def __exit__(self, type, value, traceback):
                self.o._get_lr_called_within_step = False
                return self

        with _enable_get_lr_call(self):
            for i, data in enumerate(zip(self.optimizer.param_groups, self.get_lr())):
                param_group, lr = data
                param_group['lr'] = lr
                self.print_lr(self.verbose, i, lr, epoch)

        self._last_lr = [group['lr'] for group in self.optimizer.param_groups]


class OneCycleLR(_LRScheduler):
    r"""Sets the learning rate of each parameter group according to the
    1cycle learning rate policy. The 1cycle policy anneals the learning
    rate from an initial learning rate to some maximum learning rate and then
    from that maximum learning rate to some minimum learning rate much lower
    than the initial learning rate.
    This policy was initially described in the paper `Super-Convergence:
    Very Fast Training of Neural Networks Using Large Learning Rates`_.

    The 1cycle learning rate policy changes the learning rate after every batch.
    `step` should be called after a batch has been used for training.

    This scheduler is not chainable.

    Note also that the total number of steps in the cycle can be determined in one
    of two ways (listed in order of precedence):

    #. A value for total_steps is explicitly provided.
    #. A number of epochs (epochs) and a number of steps per epoch
       (steps_per_epoch) are provided.
       In this case, the number of total steps is inferred by
       total_steps = epochs * steps_per_epoch

    You must either provide a value for total_steps or provide a value for both
    epochs and steps_per_epoch.

    The default behaviour of this scheduler follows the fastai implementation of 1cycle, which
    claims that "unpublished work has shown even better results by using only two phases". To
    mimic the behaviour of the original paper instead, set ``three_phase=True``.

    Args:
        optimizer (Optimizer): Wrapped optimizer.
        max_lr (float or list): Upper learning rate boundaries in the cycle
            for each parameter group.
        total_steps (int): The total number of steps in the cycle. Note that
            if a value is not provided here, then it must be inferred by providing
            a value for epochs and steps_per_epoch.
            Default: None
        epochs (int): The number of epochs to train for. This is used along
            with steps_per_epoch in order to infer the total number of steps in the cycle
            if a value for total_steps is not provided.
            Default: None
        steps_per_epoch (int): The number of steps per epoch to train for. This is
            used along with epochs in order to infer the total number of steps in the
            cycle if a value for total_steps is not provided.
            Default: None
        pct_start (float): The percentage of the cycle (in number of steps) spent
            increasing the learning rate.
            Default: 0.3
        anneal_strategy (str): {'cos', 'linear'}
            Specifies the annealing strategy: "cos" for cosine annealing, "linear" for
            linear annealing.
            Default: 'cos'
        cycle_momentum (bool): If ``True``, momentum is cycled inversely
            to learning rate between 'base_momentum' and 'max_momentum'.
            Default: True
        base_momentum (float or list): Lower momentum boundaries in the cycle
            for each parameter group. Note that momentum is cycled inversely
            to learning rate; at the peak of a cycle, momentum is
            'base_momentum' and learning rate is 'max_lr'.
            Default: 0.85
        max_momentum (float or list): Upper momentum boundaries in the cycle
            for each parameter group. Functionally,
            it defines the cycle amplitude (max_momentum - base_momentum).
            Note that momentum is cycled inversely
            to learning rate; at the start of a cycle, momentum is 'max_momentum'
            and learning rate is 'base_lr'
            Default: 0.95
        div_factor (float): Determines the initial learning rate via
            initial_lr = max_lr/div_factor
            Default: 25
        final_div_factor (float): Determines the minimum learning rate via
            min_lr = initial_lr/final_div_factor
            Default: 1e4
        three_phase (bool): If ``True``, use a third phase of the schedule to annihilate the
            learning rate according to 'final_div_factor' instead of modifying the second
            phase (the first two phases will be symmetrical about the step indicated by
            'pct_start').
        last_epoch (int): The index of the last batch. This parameter is used when
            resuming a training job. Since `step()` should be invoked after each
            batch instead of after each epoch, this number represents the total
            number of *batches* computed, not the total number of epochs computed.
            When last_epoch=-1, the schedule is started from the beginning.
            Default: -1
        verbose (bool): If ``True``, prints a message to stdout for
            each update. Default: ``False``.

    Example:
        >>> data_loader = torch.utils.data.DataLoader(...)
        >>> optimizer = torch.optim.SGD(model.parameters(), lr=0.1, momentum=0.9)
        >>> scheduler = torch.optim.lr_scheduler.OneCycleLR(optimizer, max_lr=0.01, steps_per_epoch=len(data_loader), epochs=10)
        >>> for epoch in range(10):
        >>>     for batch in data_loader:
        >>>         train_batch(...)
        >>>         scheduler.step()


    .. _Super-Convergence\: Very Fast Training of Neural Networks Using Large Learning Rates:
        https://arxiv.org/abs/1708.07120
    """
    def __init__(self,
                 optimizer,
                 max_lr,
                 total_steps=None,
                 epochs=None,
                 steps_per_epoch=None,
                 pct_start=0.3,
                 anneal_strategy='cos',
                 cycle_momentum=True,
                 base_momentum=0.85,
                 max_momentum=0.95,
                 div_factor=25.,
                 final_div_factor=1e4,
                 three_phase=False,
                 last_epoch=-1,
                 verbose=False):

        # Validate optimizer
        if not isinstance(optimizer, Optimizer):
            raise TypeError('{} is not an Optimizer'.format(
                type(optimizer).__name__))
        self.optimizer = optimizer

        # Validate total_steps
        if total_steps is None and epochs is None and steps_per_epoch is None:
            raise ValueError("You must define either total_steps OR (epochs AND steps_per_epoch)")
        elif total_steps is not None:
            if total_steps <= 0 or not isinstance(total_steps, int):
                raise ValueError("Expected positive integer total_steps, but got {}".format(total_steps))
            self.total_steps = total_steps
        else:
            if epochs <= 0 or not isinstance(epochs, int):
                raise ValueError("Expected positive integer epochs, but got {}".format(epochs))
            if steps_per_epoch <= 0 or not isinstance(steps_per_epoch, int):
                raise ValueError("Expected positive integer steps_per_epoch, but got {}".format(steps_per_epoch))
            self.total_steps = epochs * steps_per_epoch

        if three_phase:
            self._schedule_phases = [
                {
                    'end_step': float(pct_start * self.total_steps) - 1,
                    'start_lr': 'initial_lr',
                    'end_lr': 'max_lr',
                    'start_momentum': 'max_momentum',
                    'end_momentum': 'base_momentum',
                },
                {
                    'end_step': float(2 * pct_start * self.total_steps) - 2,
                    'start_lr': 'max_lr',
                    'end_lr': 'initial_lr',
                    'start_momentum': 'base_momentum',
                    'end_momentum': 'max_momentum',
                },
                {
                    'end_step': self.total_steps - 1,
                    'start_lr': 'initial_lr',
                    'end_lr': 'min_lr',
                    'start_momentum': 'max_momentum',
                    'end_momentum': 'max_momentum',
                },
            ]
        else:
            self._schedule_phases = [
                {
                    'end_step': float(pct_start * self.total_steps) - 1,
                    'start_lr': 'initial_lr',
                    'end_lr': 'max_lr',
                    'start_momentum': 'max_momentum',
                    'end_momentum': 'base_momentum',
                },
                {
                    'end_step': self.total_steps - 1,
                    'start_lr': 'max_lr',
                    'end_lr': 'min_lr',
                    'start_momentum': 'base_momentum',
                    'end_momentum': 'max_momentum',
                },
            ]

        # Validate pct_start
        if pct_start < 0 or pct_start > 1 or not isinstance(pct_start, float):
            raise ValueError("Expected float between 0 and 1 pct_start, but got {}".format(pct_start))

        # Validate anneal_strategy
        if anneal_strategy not in ['cos', 'linear']:
            raise ValueError("anneal_strategy must by one of 'cos' or 'linear', instead got {}".format(anneal_strategy))
        elif anneal_strategy == 'cos':
            self.anneal_func = self._annealing_cos
        elif anneal_strategy == 'linear':
            self.anneal_func = self._annealing_linear

        # Initialize learning rate variables
        max_lrs = self._format_param('max_lr', self.optimizer, max_lr)
        if last_epoch == -1:
            for idx, group in enumerate(self.optimizer.param_groups):
                group['initial_lr'] = max_lrs[idx] / div_factor
                group['max_lr'] = max_lrs[idx]
                group['min_lr'] = group['initial_lr'] / final_div_factor

        # Initialize momentum variables
        self.cycle_momentum = cycle_momentum
        if self.cycle_momentum:
            if 'momentum' not in self.optimizer.defaults and 'betas' not in self.optimizer.defaults:
                raise ValueError('optimizer must support momentum with `cycle_momentum` option enabled')
            self.use_beta1 = 'betas' in self.optimizer.defaults
            max_momentums = self._format_param('max_momentum', optimizer, max_momentum)
            base_momentums = self._format_param('base_momentum', optimizer, base_momentum)
            if last_epoch == -1:
                for m_momentum, b_momentum, group in zip(max_momentums, base_momentums, optimizer.param_groups):
                    if self.use_beta1:
                        _, beta2 = group['betas']
                        group['betas'] = (m_momentum, beta2)
                    else:
                        group['momentum'] = m_momentum
                    group['max_momentum'] = m_momentum
                    group['base_momentum'] = b_momentum

        super(OneCycleLR, self).__init__(optimizer, last_epoch, verbose)

    def _format_param(self, name, optimizer, param):
        """Return correctly formatted lr/momentum for each param group."""
        if isinstance(param, (list, tuple)):
            if len(param) != len(optimizer.param_groups):
                raise ValueError("expected {} values for {}, got {}".format(
                    len(optimizer.param_groups), name, len(param)))
            return param
        else:
            return [param] * len(optimizer.param_groups)

    def _annealing_cos(self, start, end, pct):
        "Cosine anneal from `start` to `end` as pct goes from 0.0 to 1.0."
        cos_out = math.cos(math.pi * pct) + 1
        return end + (start - end) / 2.0 * cos_out

    def _annealing_linear(self, start, end, pct):
        "Linearly anneal from `start` to `end` as pct goes from 0.0 to 1.0."
        return (end - start) * pct + start

    def get_lr(self):
        if not self._get_lr_called_within_step:
            warnings.warn("To get the last learning rate computed by the scheduler, "
                          "please use `get_last_lr()`.", UserWarning)

        lrs = []
        step_num = self.last_epoch

        if step_num > self.total_steps:
            raise ValueError("Tried to step {} times. The specified number of total steps is {}"
                             .format(step_num + 1, self.total_steps))

        for group in self.optimizer.param_groups:
            start_step = 0
            for i, phase in enumerate(self._schedule_phases):
                end_step = phase['end_step']
                if step_num <= end_step or i == len(self._schedule_phases) - 1:
                    pct = (step_num - start_step) / (end_step - start_step)
                    computed_lr = self.anneal_func(group[phase['start_lr']], group[phase['end_lr']], pct)
                    if self.cycle_momentum:
                        computed_momentum = self.anneal_func(group[phase['start_momentum']], group[phase['end_momentum']], pct)
                    break
                start_step = phase['end_step']

            lrs.append(computed_lr)
            if self.cycle_momentum:
                if self.use_beta1:
                    _, beta2 = group['betas']
                    group['betas'] = (computed_momentum, beta2)
                else:
                    group['momentum'] = computed_momentum

        return lrs