Source code for torch.nn.utils.clip_grad

# mypy: allow-untyped-decorators
# mypy: allow-untyped-defs
import functools
from typing import cast, Dict, Iterable, List, Optional, Tuple, Union
from typing_extensions import deprecated

import torch
from torch import Tensor
from torch.utils._foreach_utils import (
    _device_has_foreach_support,
    _group_tensors_by_device_and_dtype,
    _has_foreach_support,
)


__all__ = [
    "clip_grad_norm_",
    "clip_grad_norm",
    "clip_grad_value_",
]


_tensor_or_tensors = Union[torch.Tensor, Iterable[torch.Tensor]]


def _no_grad(func):
    """
    This wrapper is needed to avoid a circular import when using @torch.no_grad on the exposed functions
    clip_grad_norm_ and clip_grad_value_ themselves.
    """

    def _no_grad_wrapper(*args, **kwargs):
        with torch.no_grad():
            return func(*args, **kwargs)

    functools.update_wrapper(_no_grad_wrapper, func)
    return _no_grad_wrapper


@_no_grad
def _get_total_norm(
    tensors: _tensor_or_tensors,
    norm_type: float = 2.0,
    error_if_nonfinite: bool = False,
    foreach: Optional[bool] = None,
) -> torch.Tensor:
    r"""Compute the norm of an iterable of tensors.

    The norm is computed over the norms of the individual tensors, as if the norms of
    the individual tensors were concatenated into a single vector.

    Args:
        tensors (Iterable[Tensor] or Tensor): an iterable of Tensors or a
            single Tensor that will be normalized
        norm_type (float): type of the used p-norm. Can be ``'inf'`` for
            infinity norm.
        error_if_nonfinite (bool): if True, an error is thrown if the total
            norm of :attr:`tensors` is ``nan``, ``inf``, or ``-inf``.
            Default: ``False``
        foreach (bool): use the faster foreach-based implementation.
            If ``None``, use the foreach implementation for CUDA and CPU native tensors and silently
            fall back to the slow implementation for other device types.
            Default: ``None``

    Returns:
        Total norm of the tensors (viewed as a single vector).
    """
    if isinstance(tensors, torch.Tensor):
        tensors = [tensors]
    else:
        tensors = list(tensors)
    norm_type = float(norm_type)
    if len(tensors) == 0:
        return torch.tensor(0.0)
    first_device = tensors[0].device
    grouped_tensors: Dict[
        Tuple[torch.device, torch.dtype], Tuple[List[List[Tensor]], List[int]]
    ] = _group_tensors_by_device_and_dtype(
        [tensors]  # type: ignore[list-item]
    )  # type: ignore[assignment]

    norms: List[Tensor] = []
    for (device, _), ([device_tensors], _) in grouped_tensors.items():
        if (foreach is None and _has_foreach_support(device_tensors, device)) or (
            foreach and _device_has_foreach_support(device)
        ):
            norms.extend(torch._foreach_norm(device_tensors, norm_type))
        elif foreach:
            raise RuntimeError(
                f"foreach=True was passed, but can't use the foreach API on {device.type} tensors"
            )
        else:
            norms.extend(
                [torch.linalg.vector_norm(g, norm_type) for g in device_tensors]
            )

    total_norm = torch.linalg.vector_norm(
        torch.stack([norm.to(first_device) for norm in norms]), norm_type
    )

    if error_if_nonfinite and torch.logical_or(total_norm.isnan(), total_norm.isinf()):
        raise RuntimeError(
            f"The total norm of order {norm_type} for gradients from "
            "`parameters` is non-finite, so it cannot be clipped. To disable "
            "this error and scale the gradients by the non-finite norm anyway, "
            "set `error_if_nonfinite=False`"
        )
    return total_norm


@_no_grad
def _clip_grads_with_norm_(
    parameters: _tensor_or_tensors,
    max_norm: float,
    total_norm: torch.Tensor,
    foreach: Optional[bool] = None,
) -> None:
    r"""Scale the gradients of an iterable of parameters given a pre-calculated total norm and desired max norm.

    The gradients will be scaled by the following calculation

    .. math::
        grad = grad * \frac{max\_norm}{total\_norm + 1e-6}

    Gradients are modified in-place.

    This function is equivalent to :func:`torch.nn.utils.clip_grad_norm_` with a pre-calculated
    total norm.

    Args:
        parameters (Iterable[Tensor] or Tensor): an iterable of Tensors or a
            single Tensor that will have gradients normalized
        max_norm (float): max norm of the gradients
        total_norm (Tensor): total norm of the gradients to use for clipping
        foreach (bool): use the faster foreach-based implementation.
            If ``None``, use the foreach implementation for CUDA and CPU native tensors and silently
            fall back to the slow implementation for other device types.
            Default: ``None``

    Returns:
        None
    """
    if isinstance(parameters, torch.Tensor):
        parameters = [parameters]
    grads = [p.grad for p in parameters if p.grad is not None]
    max_norm = float(max_norm)
    if len(grads) == 0:
        return
    grouped_grads: Dict[
        Tuple[torch.device, torch.dtype], Tuple[List[List[Tensor]], List[int]]
    ] = _group_tensors_by_device_and_dtype(
        [grads]
    )  # type: ignore[assignment]

    clip_coef = max_norm / (total_norm + 1e-6)
    # Note: multiplying by the clamped coef is redundant when the coef is clamped to 1, but doing so
    # avoids a `if clip_coef < 1:` conditional which can require a CPU <=> device synchronization
    # when the gradients do not reside in CPU memory.
    clip_coef_clamped = torch.clamp(clip_coef, max=1.0)
    for (device, _), ([device_grads], _) in grouped_grads.items():
        if (foreach is None and _has_foreach_support(device_grads, device)) or (
            foreach and _device_has_foreach_support(device)
        ):
            torch._foreach_mul_(device_grads, clip_coef_clamped.to(device))
        elif foreach:
            raise RuntimeError(
                f"foreach=True was passed, but can't use the foreach API on {device.type} tensors"
            )
        else:
            clip_coef_clamped_device = clip_coef_clamped.to(device)
            for g in device_grads:
                g.mul_(clip_coef_clamped_device)


@_no_grad
def clip_grad_norm_(
    parameters: _tensor_or_tensors,
    max_norm: float,
    norm_type: float = 2.0,
    error_if_nonfinite: bool = False,
    foreach: Optional[bool] = None,
) -> torch.Tensor:
    r"""Clip the gradient norm of an iterable of parameters.

    The norm is computed over the norms of the individual gradients of all parameters,
    as if the norms of the individual gradients were concatenated into a single vector.
    Gradients are modified in-place.

    This function is equivalent to :func:`torch.nn.utils.get_total_norm` followed by
    :func:`torch.nn.utils.clip_grads_with_norm_` with the ``total_norm`` returned by ``get_total_norm``.

    Args:
        parameters (Iterable[Tensor] or Tensor): an iterable of Tensors or a
            single Tensor that will have gradients normalized
        max_norm (float): max norm of the gradients
        norm_type (float): type of the used p-norm. Can be ``'inf'`` for
            infinity norm.
        error_if_nonfinite (bool): if True, an error is thrown if the total
            norm of the gradients from :attr:`parameters` is ``nan``,
            ``inf``, or ``-inf``. Default: False (will switch to True in the future)
        foreach (bool): use the faster foreach-based implementation.
            If ``None``, use the foreach implementation for CUDA and CPU native tensors and silently
            fall back to the slow implementation for other device types.
            Default: ``None``

    Returns:
        Total norm of the parameter gradients (viewed as a single vector).
    """
    if isinstance(parameters, torch.Tensor):
        parameters = [parameters]
    else:
        # prevent generators from being exhausted
        parameters = list(parameters)
    grads = [p.grad for p in parameters if p.grad is not None]
    total_norm = _get_total_norm(grads, norm_type, error_if_nonfinite, foreach)
    _clip_grads_with_norm_(parameters, max_norm, total_norm, foreach)
    return total_norm


@deprecated(
    "`torch.nn.utils.clip_grad_norm` is now deprecated "
    "in favor of `torch.nn.utils.clip_grad_norm_`.",
    category=FutureWarning,
)
def clip_grad_norm(
    parameters: _tensor_or_tensors,
    max_norm: float,
    norm_type: float = 2.0,
    error_if_nonfinite: bool = False,
    foreach: Optional[bool] = None,
) -> torch.Tensor:
    r"""Clip the gradient norm of an iterable of parameters.

    .. warning::
        This method is now deprecated in favor of
        :func:`torch.nn.utils.clip_grad_norm_`.
    """
    return clip_grad_norm_(parameters, max_norm, norm_type, error_if_nonfinite, foreach)


@_no_grad
def clip_grad_value_(
    parameters: _tensor_or_tensors,
    clip_value: float,
    foreach: Optional[bool] = None,
) -> None:
    r"""Clip the gradients of an iterable of parameters at specified value.

    Gradients are modified in-place.

    Args:
        parameters (Iterable[Tensor] or Tensor): an iterable of Tensors or a
            single Tensor that will have gradients normalized
        clip_value (float): maximum allowed value of the gradients.
            The gradients are clipped in the range
            :math:`\left[\text{-clip\_value}, \text{clip\_value}\right]`
        foreach (bool): use the faster foreach-based implementation
            If ``None``, use the foreach implementation for CUDA and CPU native tensors and
            silently fall back to the slow implementation for other device types.
            Default: ``None``
    """
    if isinstance(parameters, torch.Tensor):
        parameters = [parameters]
    clip_value = float(clip_value)

    grads = [p.grad for p in parameters if p.grad is not None]
    grouped_grads = _group_tensors_by_device_and_dtype([grads])

    for (device, _), ([grads], _) in grouped_grads.items():
        if (
            foreach is None
            and _has_foreach_support(cast(List[Tensor], grads), device=device)
        ) or (foreach and _device_has_foreach_support(device)):
            torch._foreach_clamp_min_(cast(List[Tensor], grads), -clip_value)
            torch._foreach_clamp_max_(cast(List[Tensor], grads), clip_value)
        elif foreach:
            raise RuntimeError(
                f"foreach=True was passed, but can't use the foreach API on {device.type} tensors"
            )
        else:
            for grad in grads:
                cast(Tensor, grad).clamp_(min=-clip_value, max=clip_value)