torch/distributed/_composable/contract.py

import uuid
from collections import OrderedDict
from functools import wraps
from typing import Callable, Dict, List, Optional, Type

import torch.nn as nn
from torch.distributed._composable_state import _State


# use state_slot as key for module.__dict__ to avoid coliding with other
# properties.
# TODO: since all composable distributed features can share the same slot.
class _StateKey(str):
    # Make _StateKey as str to satisfy the assumption that object.__dict__.keys()
    # are strings.
    def __new__(cls, string="__composable_api_state_key"):
        return super().__new__(cls, f"{string}_{str(uuid.uuid4())}")


STATE_KEY = _StateKey()
REGISTRY_KEY = _StateKey()


# TODO: we can add additional info to RegistryItem to share across APIs. E.g.,
# we can add args and kwargs here, and then we can detect whether fully_shard
# is combined with reentrant activation checkpointing and error out with a clear
# message.
class RegistryItem:
    pass


def contract(state_cls: Type[_State] = _State):
    r"""
    Decorate a function as a composable distributed API, where the first
    argument of the function must be an :class:`nn.Module` instance. The
    decorator verifies that the wrapped function does not modify parameter,
    buffer or sub-module fully-qualified names (FQN).

    When a function ``func`` is decorated by ``@contract()``, a
    ``.state(module: nn.Module)`` method will be installed to the decorated
    function. Then you can retrieve and modify the state on a module by calling
    ``func.state(module)``.

    Example::
        >>> # xdoctest: +SKIP
        >>> import torch.nn as nn
        >>>
        >>> class MyModel(nn.Module):
        >>>     def __init__(self):
        >>>         super().__init__()
        >>>         self.l1 = nn.Linear(10, 10)
        >>>         self.l2 = nn.Linear(10, 10)
        >>>
        >>>     def forward(self, x):
        >>>         return self.l2(self.l1(x))
        >>>
        >>> @contract()
        >>> def my_feature(module: nn.Module) -> nn.Module:
        >>>     my_feature.state(module).some_state = "any value"
        >>>     return module
        >>>
        >>> model = MyModel()
        >>> my_feature(model.l1)
        >>> assert my_feature.state(model.l1).some_state == "any value"
        >>> my_feature(model.l2)
        >>> model(torch.randn(2, 10)).sum().backward()
    """

    # wraps will make functions decorated with contract() pickleable - needed for integration with torch.package
    @wraps(state_cls)
    def inner(func):
        @wraps(func)
        def wrapper(module: nn.Module, *args, **kwargs) -> Optional[nn.Module]:
            # get existing global states
            default_all_state: Dict[Callable, _State] = OrderedDict()
            all_state: Dict[Callable, _State] = module.__dict__.setdefault(  # type: ignore[call-overload]
                STATE_KEY, default_all_state
            )
            assert isinstance(
                all_state, dict
            ), "Distributed composable API states corrupted"

            # get global registry
            default_registry: Dict[str, RegistryItem] = OrderedDict()
            registry: Dict[str, RegistryItem] = module.__dict__.setdefault(  # type: ignore[call-overload]
                REGISTRY_KEY, default_registry
            )

            assert isinstance(
                registry, dict
            ), "Distributed composable API registry corrupted"

            # make sure the API func has not been applied to the input module yet.
            assert func not in all_state and func.__name__ not in registry, (
                "Each distinct composable distributed API can only be applied to a "
                f"module once. {func.__name__} has already been applied to the "
                f"following module.\n{module}"
            )

            # install states specific to the wrapped ``func``
            all_state.setdefault(func, state_cls())
            # register ``func`` in the global registry by name
            registry.setdefault(func.__name__, RegistryItem())

            orig_named_params = OrderedDict(module.named_parameters())
            orig_named_buffers = OrderedDict(
                module.named_buffers(remove_duplicate=False)
            )
            orig_named_modules = OrderedDict(
                module.named_modules(remove_duplicate=False)
            )

            updated = func(module, *args, **kwargs)

            if updated is None:
                updated = module

            new_named_params = OrderedDict(updated.named_parameters())
            new_named_buffers = OrderedDict(
                updated.named_buffers(remove_duplicate=False)
            )
            new_named_modules = OrderedDict(
                updated.named_modules(remove_duplicate=False)
            )

            assert isinstance(updated, nn.Module), (
                "Output of composable distributed APIs must be either None or "
                f"nn.Module, but got {type(updated)}"
            )

            def check_fqn(orig_fqns: List[str], new_fqns: List[str]):
                if orig_fqns == new_fqns:
                    return

                orig_fqn_set, new_fqn_set = set(orig_fqns), set(new_fqns)
                orig_only = orig_fqn_set - new_fqn_set
                new_only = new_fqn_set - orig_fqn_set
                if len(orig_only) or len(new_only):
                    raise RuntimeError(
                        "Composable distributed API implementations cannot modify "
                        "FQNs.\n"
                        f"Only in original FQNs: {orig_only},\n"
                        f"Only in new FQNs: {new_only}"
                    )
                else:
                    raise RuntimeError(
                        "Composable distributed API implementations cannot modify "
                        "the order of FQNs.\n"
                        f"Original FQNs: {orig_only}\n"
                        f"New FQNs: {new_only}"
                    )

            check_fqn(list(orig_named_params.keys()), list(new_named_params.keys()))
            check_fqn(list(orig_named_buffers.keys()), list(new_named_buffers.keys()))
            check_fqn(list(orig_named_modules.keys()), list(new_named_modules.keys()))

            # TODO: a stricter verification should also reject changing module
            # types and monkey-patching forward() method implementations.

            # TODO: verify that installed distributed paradigms are compatible with
            # each other.

            return updated

        def get_state(module: nn.Module) -> Optional[_State]:
            return module.__dict__.setdefault(  # type: ignore[call-overload]
                STATE_KEY,
                {},  # TODO(@yhcharles): this is a temporary fix, need a better way
            ).get(
                func
            )  # type: ignore[call-overload]

        wrapper.state = get_state  # type: ignore[attr-defined]

        return wrapper

    return inner


def _get_registry(module: nn.Module) -> Dict[str, RegistryItem]:
    r"""
    Get an ``OrderedDict`` of composable APIs that have been applied to the
    ``module``, indexed by the API name.
    """
    default_registry: Dict[str, RegistryItem] = OrderedDict()
    return module.__dict__.setdefault(REGISTRY_KEY, default_registry)  # type: ignore[call-overload]