Source code for torch.distributed.pipelining.microbatch

# mypy: allow-untyped-defs
# Copyright (c) Meta Platforms, Inc. and affiliates
import logging
from typing import Any, Dict, List, Optional, Tuple

import torch
from torch.fx.node import map_aggregate
from torch.utils._pytree import tree_flatten, tree_unflatten


__all__ = [
    "TensorChunkSpec",
    "split_args_kwargs_into_chunks",
    "merge_chunks",
]

logger = logging.getLogger(__name__)

"""
_debug_mask_minibatches specifies to send masked versions of the mini-batch
through instead of micro-batch slices--this can be used for more stable
numerical testing (see [A Note About Correctness Testing])
"""
_debug_mask_minibatches = False


class _CustomReducer:
    """
    Custom reducer class that can be used to specify a custom operation that
    reduces losses of multiple microbatches into one value.

    Example:
    >>> # xdoctest: +SKIP
    >>> sum_reducer = _CustomReducer(
    >>>     torch.tensor(0.0),
    >>>     lambda a, b: a + b
    >>> )
    """

    def __init__(self, init_value, reduce_fn):
        self.init_value = init_value
        self.reduce_fn = reduce_fn


class _LossReducer(_CustomReducer):
    pass


sum_reducer = _LossReducer(torch.tensor(0.0), lambda a, b: a + b)

# Default chunking dimension is 0. This is used for the case where the user did
# not specify a chunking dimension.
DEFAULT_CHUNK_DIM = 0


class TensorChunkSpec:
    """
    Class used to specify chunking of inputs
    """

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

    split_dim: int

    def __repr__(self):
        return (
            f"{self.__class__.__module__}.{self.__class__.__name__}({self.split_dim})"
        )

    def __str__(self):
        return f"TensorChunkSpec({self.split_dim})"

    @staticmethod
    def from_tuple(
        chunk_dims: Tuple[int, ...],
    ):
        """
        A helper for creating a tuple of `TensorChunkSpec` from a tuple of chunk
        dimensions (int's).
        Example:
            >>> # xdoctest: +SKIP
            >>> # There are three positional arguments to the model, and
            >>> # we are chunking them along dimension 0, 0 and 1, respectively
            >>> args_chunk_spec = TensorChunkSpec.from_tuple((0, 0, 1))
        """
        args_chunk_spec = map_aggregate(
            chunk_dims,
            lambda dim: TensorChunkSpec(dim),  # type: ignore[arg-type,return-value]
        )
        return args_chunk_spec

    @staticmethod
    def from_dict(
        chunk_dims: Dict[str, int],
    ):
        """
        A helper for creating a dictionary of `TensorChunkSpec` from a
        dictionary of chunk dimensions (int's).
        Example:
            >>> # xdoctest: +SKIP
            >>> # Chunk dimension 0 for the "id" argument, 1 for the "mask" argument
            >>> kwargs_chunk_spec = TensorChunkSpec.from_dict({"id": 0, "mask": 1})
        """
        kwargs_chunk_spec = map_aggregate(
            chunk_dims,
            lambda dim: TensorChunkSpec(dim),  # type: ignore[arg-type,return-value]
        )
        return kwargs_chunk_spec


# Class used to specify replication of inputs
class _Replicate:
    pass


def _shard_dict_of_args(
    args_dict,
    args_chunk_spec,
    num_chunks,
):
    """
    Given a dictionary of args, and a dictionary of chunking specs, shard the
    args according to the chunking specs.

    Args:
        args_dict: Dictionary of args
        args_chunk_spec: Dictionary of chunking specs
        num_chunks: Number of chunks to shard the args into

    Returns:
        args_split: List of sharded args
    """
    # Stage 1+2: flatten and shard/replicate

    # args_sharded_replicated : [num args, num flat values, num chunks]
    args_sharded_replicated = {}
    arg_specs = []

    real_num_chunks = num_chunks
    first_tensor = True

    assert len(args_dict) == len(
        args_chunk_spec
    ), f"args_dict.keys() = {list(args_dict.keys())} args_chunk_spec.keys() = {list(args_chunk_spec.keys())}"

    for arg_key, arg in args_dict.items():
        flat, spec = tree_flatten(arg)
        arg_specs.append(spec)

        chunk_spec = args_chunk_spec[arg_key]
        assert chunk_spec is not None  # Should have been set by caller
        chunk_spec_flat, _ = tree_flatten(chunk_spec)
        if len(flat) != len(chunk_spec_flat):
            raise ValueError(
                f"Argument value {arg} did not have the same number of "
                f"values as as chunk spec {chunk_spec}"
            )

        sharded_arg_flat = []

        for v, chunk_v in zip(flat, chunk_spec_flat):
            if chunk_v is _Replicate or not isinstance(v, torch.Tensor):
                sharded_arg_flat.append([v] * real_num_chunks)
            elif isinstance(chunk_v, TensorChunkSpec):
                # TODO: check type of v. If it's a tensor, use chunk (or debug mask).
                # If it's a collection type, split it as you would expect. Otherwise,
                # Throw an error
                assert isinstance(v, torch.Tensor), f"{v} is not a tensor"

                v_split_dim_size = v.size(chunk_v.split_dim)
                if v_split_dim_size < real_num_chunks:
                    if first_tensor:
                        # We can only adjust number of chunks when we hit this
                        # issue at the first tensor encountered
                        logger.warning(
                            f"Tensor size on chunking dimension is {v_split_dim_size}, "  # noqa: G004
                            f"downsizing the number of chunks from {num_chunks} to {v_split_dim_size}."
                        )
                        real_num_chunks = v_split_dim_size
                    else:
                        raise RuntimeError(
                            f"Arg {arg_key} on chunking dimension has a size of {v_split_dim_size}, "
                            f"smaller than the number of chunks {num_chunks}. "
                            "PiPPy cannot reduce the number of chunks because "
                            "other arguments have bigger chunk-dimension sizes. "
                            "Please adjust your num_chunks setting."
                        )

                chunk_tensors = torch.tensor_split(
                    v, real_num_chunks, chunk_v.split_dim
                )

                if _debug_mask_minibatches:
                    expanded_chunks = []

                    split_dim_idx = 0
                    for chunk_tensor in chunk_tensors:
                        new_val = torch.zeros_like(v)
                        upper_idx = split_dim_idx + chunk_tensor.size(chunk_v.split_dim)

                        slice_indices = [slice(None, None, None)] * new_val.ndim
                        slice_indices[chunk_v.split_dim] = slice(
                            split_dim_idx, upper_idx
                        )
                        new_val[slice_indices] = chunk_tensor

                        expanded_chunks.append(new_val)

                        split_dim_idx += chunk_tensor.size(chunk_v.split_dim)

                    sharded_arg_flat.append(expanded_chunks)
                else:
                    sharded_arg_flat.append(chunk_tensors)  # type: ignore[arg-type]

                first_tensor = False
            else:
                raise TypeError(f"Unrecognized chunk spec: {chunk_v}")

        args_sharded_replicated[arg_key] = sharded_arg_flat

    # chunks_flat : [num chunks, num args, num flat values]
    chunks_flat = []
    for chunk_idx in range(real_num_chunks):
        chunk_args = {}
        for key, arg in args_sharded_replicated.items():
            arg_single_chunk = [v_flat[chunk_idx] for v_flat in arg]
            chunk_args[key] = arg_single_chunk
        chunks_flat.append(chunk_args)

    # args_split : [num chunks, num args]
    args_split = []

    for chunk in chunks_flat:
        per_chunk_args = {}
        assert len(arg_specs) == len(chunk)
        for (key, arg), arg_spec in zip(chunk.items(), arg_specs):
            per_chunk_args[key] = tree_unflatten(arg, arg_spec)
        args_split.append(per_chunk_args)

    return args_split


def split_args_kwargs_into_chunks(
    args: Tuple[Any, ...],
    kwargs: Optional[Dict[str, Any]],
    chunks: int,
    args_chunk_spec: Optional[Tuple[TensorChunkSpec, ...]] = None,
    kwargs_chunk_spec: Optional[Dict[str, TensorChunkSpec]] = None,
) -> Tuple[List[Tuple], List[Dict]]:
    """
    Given a sequence of args and kwargs, split them into a number of chunks
    according to  their respective chunking specs.

    Args:
        args: Tuple of args
        kwargs: Dict of kwargs
        chunks: Number of chunks to split the args and kwargs into
        args_chunk_spec: chunking specs for args, in same shape as args
        kwargs_chunk_spec: chunking specs for kwargs, in same shape as kwargs

    Returns:
        args_split: List of sharded args
        kwargs_split: List of sharded kwargs
    """
    # Given `args` and `kwargs`, we want to yield a set of `chunks` args and kwargs such that
    # the constituent Tensor values have been sharded/replicated according to the `args_chunk_spec`
    # and `kwargs_chunk_spec` specifications. The steps are as follows:
    #
    # 1. Use pytree.tree_flatten to flatten each arg and its spec into nto a 1d array of values.
    #    To use a running example: suppose our inputs look like
    #
    #       args = ([A, [B, C]], D) args_spec = ([None, [None, TensorChunkSpec]], None)
    #       (kwargs not shown but it's a similar process)
    #
    #    Then for this step we would end up with
    #
    #       args = ([A, B, C], D) args_spec = ([None, None, TensorChunkSpec], None)
    #
    # 2. Shard or replicate the arguments subject to the policy in the spec. Suppose chunks = 2
    #
    #       args = ([[A, A], [B, B], [C_1, C_2]], [D, D])
    #
    # 3. Rotate the nesting order such that chunks are the outer dimension
    #
    #       args_chunks = [
    #           ([A, B, C_1], D),
    #           ([A, B, C_2], D),
    #       ]
    #
    # 4. Unflatten each chunk according to the spec
    #
    #       args_chunks = [
    #           ([A, [B, C_1]], D),
    #           ([A, [B, C_2]], D),
    #       ]

    # TODO: _debug_mask_minibatches
    # Handle the case where kwargs is None
    if kwargs is None:
        kwargs = {}

    # If user did not provide args_chunk_spec or kwargs_chunk_spec, we extend
    # their format and use default chunking along dim 0
    if args_chunk_spec is None:
        args_chunk_spec = (TensorChunkSpec(DEFAULT_CHUNK_DIM),) * len(args)

    if kwargs_chunk_spec is None:
        kwargs_chunk_spec = dict.fromkeys(kwargs, TensorChunkSpec(DEFAULT_CHUNK_DIM))

    args_split_dict = _shard_dict_of_args(
        dict(enumerate(args)),
        dict(enumerate(args_chunk_spec)),
        chunks,
    )
    real_num_chunks = len(args_split_dict)

    kwargs_split = _shard_dict_of_args(
        kwargs,
        kwargs_chunk_spec,
        real_num_chunks,
    )

    if len(kwargs_split) < real_num_chunks:
        # In case kwargs are sharded into less chunks
        # e.g. when `args` has no tensor, just values
        real_num_chunks = len(kwargs_split)
        # Re-shard args
        args_split_dict = _shard_dict_of_args(
            dict(enumerate(args)),
            dict(enumerate(args_chunk_spec)),
            real_num_chunks,
        )

    if len(args_split_dict) != len(kwargs_split):
        raise RuntimeError(
            "args and kwargs are split into different number of chunks: "
            f"{len(args_split_dict)}, {len(kwargs_split)}"
        )

    args_split = [
        tuple(chunk_args[i] for i in range(len(chunk_args)))
        for chunk_args in args_split_dict
    ]

    return args_split, kwargs_split


def merge_chunks(
    chunks: List[Any],
    chunk_spec,
):
    """
    Given a list of chunks, merge them into a single value according to
    the chunk spec.

    Args:
        chunks: list of chunks
        chunk_spec: Chunking spec for the chunks

    Returns:
        value: Merged value
    """
    # This is essentially the inverse of `split_args_kwargs_into_chunks`, so the
    # steps are similar to the steps in that function but in reverse. Given the
    # input values:
    #
    #       chunks = [
    #           ([A, [B, C_1]], D),
    #           ([A, [B, C_2]], D),
    #       ]
    #       args_spec = ([None, [None, TensorChunkSpec]], None)
    #
    # 1. Flatten the chunks according to the chunk_spec
    #
    #       chunks_flat = [
    #           ([A, B, C_1], D),
    #           ([A, B, C_2], D),
    #       ]
    #
    # 2. Rotate the nesting order such that chunks are the inner dimension
    #
    #       value_inner = ([A, B, [C_1, C_2]], D)
    #
    # 3. Concatenate sharded arguments
    #
    #       value_combined = ([A, B, C], D)
    #
    # 4. Unflatten the combined args given the spec
    #
    #       value = ([A, [B, C]], D)

    # Preliminary: flatten the chunk spec
    if chunk_spec is not None:
        spec_flattened, flatten_spec = tree_flatten(chunk_spec)
    else:
        # If chunk_spec is not provided, we will merge chunks along the default dimension (0), for all output fields
        # We obtain the output structure by flattening chunk 0 and generate the chunk_spec
        chunk0_flat, flatten_spec = tree_flatten(chunks[0])
        spec_flattened = [TensorChunkSpec(DEFAULT_CHUNK_DIM)] * len(chunk0_flat)

    # Stage 1: flatten chunks
    # chunks_flattened : [num chunks, num args]
    chunks_flattened = []

    for chunk in chunks:
        chunk_flattened, _ = tree_flatten(chunk)
        if len(chunk_flattened) != len(spec_flattened):
            raise ValueError(f"Chunk {chunk} did not match chunk spec {chunk_spec}")

        chunks_flattened.append(chunk_flattened)

    # Stage 2 and 3: Rotate nesting order s.t. chunks are inner dimension and
    #                concatenate sharded operands
    # args_flattened : [num args]
    args_flattened = []
    for arg_idx, arg in enumerate(spec_flattened):
        if isinstance(arg, TensorChunkSpec):
            partial_values = [
                chunks_flattened[chunk_idx][arg_idx]
                for chunk_idx in range(len(chunks_flattened))
            ]

            if _debug_mask_minibatches:
                # Infer size of individual chunks by running `tensor_split` again
                overall_shape = partial_values[0].shape
                for val in partial_values[1:]:
                    assert val.shape == overall_shape
                meta_chunks = torch.tensor_split(
                    torch.empty(*overall_shape, device="meta"),
                    sections=len(partial_values),
                    dim=arg.split_dim,
                )

                values_to_cat = []
                chunk_start_idx = 0
                assert len(partial_values) == len(meta_chunks)
                for partial_value, meta_chunk in zip(partial_values, meta_chunks):
                    chunk_end_idx = chunk_start_idx + meta_chunk.size(arg.split_dim)

                    slice_indices = [slice(None, None, None)] * partial_value.ndim
                    slice_indices[arg.split_dim] = slice(chunk_start_idx, chunk_end_idx)
                    sliced = partial_value[slice_indices]
                    values_to_cat.append(sliced)

                    chunk_start_idx = chunk_end_idx

            else:
                values_to_cat = partial_values

            args_flattened.append(torch.cat(values_to_cat, dim=arg.split_dim))
        elif isinstance(arg, _CustomReducer):
            reduced_val = arg.init_value

            for chunk_idx in range(len(chunks_flattened)):
                reduced_val = arg.reduce_fn(
                    reduced_val, chunks_flattened[chunk_idx][arg_idx]
                )

            args_flattened.append(reduced_val)
        else:
            value = chunks_flattened[0][arg_idx]
            for chunk_idx in range(1, len(chunks_flattened)):
                assert chunks_flattened[chunk_idx][arg_idx] == value
            args_flattened.append(value)

    # Stage 4: Unflatten combined args
    return tree_unflatten(args_flattened, flatten_spec)