_vmap_internals.py

import torch
import functools
from torch import Tensor
from typing import Any, Callable, Optional, Tuple, Union, List
from torch.utils._pytree import tree_flatten, tree_unflatten, _broadcast_to_and_flatten
import warnings

in_dims_t = Union[int, Tuple]
out_dims_t = Union[int, Tuple[int, ...]]

# Checks that all args-to-be-batched have the same batch dim size
def _validate_and_get_batch_size(
        flat_in_dims: List[Optional[int]],
        flat_args: List) -> int:
    batch_sizes = [arg.size(in_dim) for in_dim, arg in zip(flat_in_dims, flat_args)
                   if in_dim is not None]
    if batch_sizes and any([size != batch_sizes[0] for size in batch_sizes]):
        raise ValueError(
            f'vmap: Expected all tensors to have the same size in the mapped '
            f'dimension, got sizes {batch_sizes} for the mapped dimension')
    return batch_sizes[0]

def _num_outputs(batched_outputs: Union[Tensor, Tuple[Tensor, ...]]) -> int:
    if isinstance(batched_outputs, tuple):
        return len(batched_outputs)
    return 1

# If value is a tuple, check it has length `num_elements`.
# If value is not a tuple, make a tuple with `value` repeated `num_elements` times
def _as_tuple(value: Any, num_elements: int, error_message_lambda: Callable[[], str]) -> Tuple:
    if not isinstance(value, tuple):
        return (value,) * num_elements
    if len(value) != num_elements:
        raise ValueError(error_message_lambda())
    return value

# Creates BatchedTensors for every Tensor in arg that should be batched.
# Returns the (potentially) batched arguments and the batch_size.
def _create_batched_inputs(
        in_dims: in_dims_t, args: Tuple, vmap_level: int, func: Callable) -> Tuple[Tuple, int]:
    if not isinstance(in_dims, int) and not isinstance(in_dims, tuple):
        raise ValueError(
            f'vmap({_get_name(func)}, in_dims={in_dims}, ...)(<inputs>): '
            f'expected `in_dims` to be int or a (potentially nested) tuple '
            f'matching the structure of inputs, got: {type(in_dims)}.')
    if len(args) == 0:
        raise ValueError(
            f'vmap({_get_name(func)})(<inputs>): got no inputs. Maybe you forgot to add '
            f'inputs, or you are trying to vmap over a function with no inputs. '
            f'The latter is unsupported.')

    flat_args, args_spec = tree_flatten(args)
    flat_in_dims = _broadcast_to_and_flatten(in_dims, args_spec)
    if flat_in_dims is None:
        raise ValueError(
            f'vmap({_get_name(func)}, in_dims={in_dims}, ...)(<inputs>): '
            f'in_dims is not compatible with the structure of `inputs`. '
            f'in_dims has structure {tree_flatten(in_dims)[1]} but inputs '
            f'has structure {args_spec}.')

    for arg, in_dim in zip(flat_args, flat_in_dims):
        if not isinstance(in_dim, int) and in_dim is not None:
            raise ValueError(
                f'vmap({_get_name(func)}, in_dims={in_dims}, ...)(<inputs>): '
                f'Got in_dim={in_dim} for an input but in_dim must be either '
                f'an integer dimension or None.')
        if isinstance(in_dim, int) and not isinstance(arg, Tensor):
            raise ValueError(
                f'vmap({_get_name(func)}, in_dims={in_dims}, ...)(<inputs>): '
                f'Got in_dim={in_dim} for an input but the input is of type '
                f'{type(arg)}. We cannot vmap over non-Tensor arguments, '
                f'please use None as the respective in_dim')
        if in_dim is not None and (in_dim < 0 or in_dim >= arg.dim()):
            raise ValueError(
                f'vmap({_get_name(func)}, in_dims={in_dims}, ...)(<inputs>): '
                f'Got in_dim={in_dim} for some input, but that input is a Tensor '
                f'of dimensionality {arg.dim()} so expected in_dim to satisfy '
                f'0 <= in_dim < {arg.dim()}.')

    batch_size = _validate_and_get_batch_size(flat_in_dims, flat_args)
    # See NOTE [Ignored _remove_batch_dim, _add_batch_dim]
    batched_inputs = [arg if in_dim is None else
                      torch._add_batch_dim(arg, in_dim, vmap_level)
                      for in_dim, arg in zip(flat_in_dims, flat_args)]
    return tree_unflatten(batched_inputs, args_spec), batch_size

# Undos the batching (and any batch dimensions) associated with the `vmap_level`.
def _unwrap_batched(
        batched_outputs: Union[Tensor, Tuple[Tensor, ...]],
        out_dims: out_dims_t,
        vmap_level: int, batch_size: int, func: Callable) -> Tuple:
    num_outputs = _num_outputs(batched_outputs)
    out_dims_as_tuple = _as_tuple(
        out_dims, num_outputs,
        lambda: f'vmap({_get_name(func)}, ..., out_dims={out_dims}): `out_dims` must '
                f'have one dim per output (got {num_outputs} outputs) of {_get_name(func)}.')

    # NOTE [Ignored _remove_batch_dim, _add_batch_dim]
    # There is something wrong with our type bindings for functions that begin
    # with '_', see #40397.
    if isinstance(batched_outputs, Tensor):
        out_dim = out_dims_as_tuple[0]
        return torch._remove_batch_dim(batched_outputs, vmap_level, batch_size, out_dim)  # type: ignore[return-value]
    return tuple(torch._remove_batch_dim(out, vmap_level, batch_size, out_dim)
                 for out, out_dim in zip(batched_outputs, out_dims_as_tuple))

# Checks that `fn` returned one or more Tensors and nothing else.
# NB: A python function that return multiple arguments returns a single tuple,
# so we are effectively checking that `outputs` is a single Tensor or a tuple of
# Tensors.
def _validate_outputs(outputs: Any, func: Callable) -> None:
    if isinstance(outputs, Tensor):
        return
    if not isinstance(outputs, tuple):
        raise ValueError(f'vmap({_get_name(func)}, ...): `{_get_name(func)}` must only return '
                         f'Tensors, got type {type(outputs)} as the return.')
    for idx, output in enumerate(outputs):
        if isinstance(output, Tensor):
            continue
        raise ValueError(f'vmap({_get_name(func)}, ...): `{_get_name(func)}` must only return '
                         f'Tensors, got type {type(output)} for return {idx}.')

def _check_out_dims_is_int_or_int_tuple(out_dims: out_dims_t, func: Callable) -> None:
    if isinstance(out_dims, int):
        return
    if not isinstance(out_dims, tuple) or \
            not all([isinstance(out_dim, int) for out_dim in out_dims]):
        raise ValueError(
            f'vmap({_get_name(func)}, ..., out_dims={out_dims}): `out_dims` must be '
            f'an int or a tuple of int representing where in the outputs the '
            f'vmapped dimension should appear.')

def _get_name(func: Callable):
    if hasattr(func, '__name__'):
        return func.__name__

    # Not all callables have __name__, in fact, only static functions/methods do.
    # A callable created via functools.partial or an nn.Module, to name some
    # examples, don't have a __name__.
    return repr(func)

# vmap(func)(inputs) wraps all Tensor inputs to be batched in BatchedTensors,
# sends those into func, and then unwraps the output BatchedTensors. Operations
# on BatchedTensors perform the batched operations that the user is asking for.
def vmap(func: Callable, in_dims: in_dims_t = 0, out_dims: out_dims_t = 0) -> Callable:
    """
    vmap is the vectorizing map. Returns a new function that maps `func` over some
    dimension of the inputs. Semantically, vmap pushes the map into PyTorch
    operations called by `func`, effectively vectorizing those operations.

    vmap is useful for handling batch dimensions: one can write a function `func`
    that runs on examples and then lift it to a function that can take batches of
    examples with `vmap(func)`. vmap can also be used to compute batched
    gradients when composed with autograd.

    .. note::
        We are actively developing a different and improved vmap prototype
        `here. <https://github.com/zou3519/functorch>`_ The improved
        prototype is able to arbitrarily compose with gradient computation.
        Please give that a try if that is what you're looking for.

        Furthermore, if you're interested in using vmap for your use case,
        please `contact us! <https://github.com/pytorch/pytorch/issues/42368>`_
        We're interested in gathering feedback from early adopters to inform
        the design.

    .. warning::
        torch.vmap is an experimental prototype that is subject to
        change and/or deletion. Please use at your own risk.

    Args:
        func (function): A Python function that takes one or more arguments.
            Must return one or more Tensors.
        in_dims (int or nested structure): Specifies which dimension of the
            inputs should be mapped over. `in_dims` should have a structure
            like the inputs. If the `in_dim` for a particular input is None,
            then that indicates there is no map dimension. Default: 0.
        out_dims (int or Tuple[int]): Specifies where the mapped dimension
            should appear in the outputs. If `out_dims` is a Tuple, then it should
            have one element per output. Default: 0.

    Returns:
        Returns a new "batched" function. It takes the same inputs as `func`,
        except each input has an extra dimension at the index specified by `in_dims`.
        It takes returns the same outputs as `func`, except each output has
        an extra dimension at the index specified by `out_dims`.

    .. warning:
        vmap works best with functional-style code. Please do not perform any
        side-effects in `func`, with the exception of in-place PyTorch operations.
        Examples of side-effects include mutating Python data structures and
        assigning values to variables not captured in `func`.

    One example of using `vmap` is to compute batched dot products. PyTorch
    doesn't provide a batched `torch.dot` API; instead of unsuccessfully
    rummaging through docs, use `vmap` to construct a new function.

        >>> torch.dot                            # [D], [D] -> []
        >>> batched_dot = torch.vmap(torch.dot)  # [N, D], [N, D] -> [N]
        >>> x, y = torch.randn(2, 5), torch.randn(2, 5)
        >>> batched_dot(x, y)

    `vmap` can be helpful in hiding batch dimensions, leading to a simpler
    model authoring experience.

        >>> batch_size, feature_size = 3, 5
        >>> weights = torch.randn(feature_size, requires_grad=True)
        >>>
        >>> def model(feature_vec):
        >>>     # Very simple linear model with activation
        >>>     return feature_vec.dot(weights).relu()
        >>>
        >>> examples = torch.randn(batch_size, feature_size)
        >>> result = torch.vmap(model)(examples)

    `vmap` can also help vectorize computations that were previously difficult
    or impossible to batch. One example is higher-order gradient computation.
    The PyTorch autograd engine computes vjps (vector-Jacobian products).
    Computing a full Jacobian matrix for some function f: R^N -> R^N usually
    requires N calls to `autograd.grad`, one per Jacobian row. Using `vmap`,
    we can vectorize the whole computation, computing the Jacobian in a single
    call to `autograd.grad`.

        >>> # Setup
        >>> N = 5
        >>> f = lambda x: x ** 2
        >>> x = torch.randn(N, requires_grad=True)
        >>> y = f(x)
        >>> I_N = torch.eye(N)
        >>>
        >>> # Sequential approach
        >>> jacobian_rows = [torch.autograd.grad(y, x, v, retain_graph=True)[0]
        >>>                  for v in I_N.unbind()]
        >>> jacobian = torch.stack(jacobian_rows)
        >>>
        >>> # vectorized gradient computation
        >>> def get_vjp(v):
        >>>     return torch.autograd.grad(y, x, v)
        >>> jacobian = torch.vmap(get_vjp)(I_N)

    .. note::
        vmap does not provide general autobatching or handle variable-length
        sequences out of the box.
    """
    warnings.warn(
        'torch.vmap is an experimental prototype that is subject to '
        'change and/or deletion. Please use at your own risk. There may be '
        'unexpected performance cliffs due to certain operators not being '
        'implemented. To see detailed performance warnings please use '
        '`torch._C._debug_only_display_vmap_fallback_warnings(True) '
        'before the call to `vmap`.',
        stacklevel=2)
    return _vmap(func, in_dims, out_dims)

# A version of vmap but without the initial "experimental prototype" warning
def _vmap(func: Callable, in_dims: in_dims_t = 0, out_dims: out_dims_t = 0) -> Callable:
    @functools.wraps(func)
    def wrapped(*args):
        _check_out_dims_is_int_or_int_tuple(out_dims, func)
        vmap_level = torch._C._vmapmode_increment_nesting()
        try:
            batched_inputs, batch_size = _create_batched_inputs(in_dims, args, vmap_level, func)
            batched_outputs = func(*batched_inputs)
            _validate_outputs(batched_outputs, func)
            return _unwrap_batched(batched_outputs, out_dims, vmap_level, batch_size, func)
        finally:
            torch._C._vmapmode_decrement_nesting()
    return wrapped