# Copyright 2025 The HuggingFace Team. All rights reserved.

#

# Licensed under the Apache License, Version 2.0 (the "License");

# you may not use this file except in compliance with the License.

# You may obtain a copy of the License at

#

# http://www.apache.org/licenses/LICENSE-2.0

#

# Unless required by applicable law or agreed to in writing, software

# distributed under the License is distributed on an "AS IS" BASIS,

# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

# See the License for the specific language governing permissions and

# limitations under the License.

import inspect

from dataclasses import dataclass

from typing import Dict, List, Type, Union

import torch

if torch.distributed.is_available():

import torch.distributed._functional_collectives as funcol

from ..models._modeling_parallel import (

ContextParallelConfig,

ContextParallelInput,

ContextParallelModelPlan,

ContextParallelOutput,

)

from ..utils import get_logger

from ..utils.torch_utils import unwrap_module

from .hooks import HookRegistry, ModelHook

logger = get_logger(__name__) # pylint: disable=invalid-name

_CONTEXT_PARALLEL_INPUT_HOOK_TEMPLATE = "cp_input---{}"

_CONTEXT_PARALLEL_OUTPUT_HOOK_TEMPLATE = "cp_output---{}"

# TODO(aryan): consolidate with ._helpers.TransformerBlockMetadata

@dataclass

class ModuleForwardMetadata:

cached_parameter_indices: Dict[str, int] = None

_cls: Type = None

def _get_parameter_from_args_kwargs(self, identifier: str, args=(), kwargs=None):

kwargs = kwargs or {}

if identifier in kwargs:

return kwargs[identifier], True, None

if self.cached_parameter_indices is not None:

index = self.cached_parameter_indices.get(identifier, None)

if index is None:

raise ValueError(f"Parameter '{identifier}' not found in cached indices.")

return args[index], False, index

if self._cls is None:

raise ValueError("Model class is not set for metadata.")

parameters = list(inspect.signature(self._cls.forward).parameters.keys())

parameters = parameters[1:] # skip `self`

self.cached_parameter_indices = {param: i for i, param in enumerate(parameters)}

if identifier not in self.cached_parameter_indices:

raise ValueError(f"Parameter '{identifier}' not found in function signature but was requested.")

index = self.cached_parameter_indices[identifier]

if index >= len(args):

raise ValueError(f"Expected {index} arguments but got {len(args)}.")

return args[index], False, index

def apply_context_parallel(

module: torch.nn.Module,

parallel_config: ContextParallelConfig,

plan: Dict[str, ContextParallelModelPlan],

) -> None:

"""Apply context parallel on a model."""

logger.debug(f"Applying context parallel with CP mesh: {parallel_config._mesh} and plan: {plan}")

for module_id, cp_model_plan in plan.items():

submodule = _get_submodule_by_name(module, module_id)

if not isinstance(submodule, list):

submodule = [submodule]

logger.debug(f"Applying ContextParallelHook to {module_id=} identifying a total of {len(submodule)} modules")

for m in submodule:

if isinstance(cp_model_plan, dict):

hook = ContextParallelSplitHook(cp_model_plan, parallel_config)

hook_name = _CONTEXT_PARALLEL_INPUT_HOOK_TEMPLATE.format(module_id)

elif isinstance(cp_model_plan, (ContextParallelOutput, list, tuple)):

if isinstance(cp_model_plan, ContextParallelOutput):

cp_model_plan = [cp_model_plan]

if not all(isinstance(x, ContextParallelOutput) for x in cp_model_plan):

raise ValueError(f"Expected all elements of cp_model_plan to be CPOutput, but got {cp_model_plan}")

hook = ContextParallelGatherHook(cp_model_plan, parallel_config)

hook_name = _CONTEXT_PARALLEL_OUTPUT_HOOK_TEMPLATE.format(module_id)

else:

raise ValueError(f"Unsupported context parallel model plan type: {type(cp_model_plan)}")

registry = HookRegistry.check_if_exists_or_initialize(m)

registry.register_hook(hook, hook_name)

def remove_context_parallel(module: torch.nn.Module, plan: Dict[str, ContextParallelModelPlan]) -> None:

for module_id, cp_model_plan in plan.items():

submodule = _get_submodule_by_name(module, module_id)

if not isinstance(submodule, list):

submodule = [submodule]

for m in submodule:

registry = HookRegistry.check_if_exists_or_initialize(m)

if isinstance(cp_model_plan, dict):

hook_name = _CONTEXT_PARALLEL_INPUT_HOOK_TEMPLATE.format(module_id)

elif isinstance(cp_model_plan, (ContextParallelOutput, list, tuple)):

hook_name = _CONTEXT_PARALLEL_OUTPUT_HOOK_TEMPLATE.format(module_id)

else:

raise ValueError(f"Unsupported context parallel model plan type: {type(cp_model_plan)}")

registry.remove_hook(hook_name)

class ContextParallelSplitHook(ModelHook):

def __init__(self, metadata: ContextParallelModelPlan, parallel_config: ContextParallelConfig) -> None:

super().__init__()

self.metadata = metadata

self.parallel_config = parallel_config

self.module_forward_metadata = None

def initialize_hook(self, module):

cls = unwrap_module(module).__class__

self.module_forward_metadata = ModuleForwardMetadata(_cls=cls)

return module

def pre_forward(self, module, *args, **kwargs):

args_list = list(args)

for name, cpm in self.metadata.items():

if isinstance(cpm, ContextParallelInput) and cpm.split_output:

continue

# Maybe the parameter was passed as a keyword argument

input_val, is_kwarg, index = self.module_forward_metadata._get_parameter_from_args_kwargs(

name, args_list, kwargs

)

if input_val is None:

continue

# The input_val may be a tensor or list/tuple of tensors. In certain cases, user may specify to shard

# the output instead of input for a particular layer by setting split_output=True

if isinstance(input_val, torch.Tensor):

input_val = self._prepare_cp_input(input_val, cpm)

elif isinstance(input_val, (list, tuple)):

if len(input_val) != len(cpm):

raise ValueError(

f"Expected input model plan to have {len(input_val)} elements, but got {len(cpm)}."

)

sharded_input_val = []

for i, x in enumerate(input_val):

if torch.is_tensor(x) and not cpm[i].split_output:

x = self._prepare_cp_input(x, cpm[i])

sharded_input_val.append(x)

input_val = sharded_input_val

else:

raise ValueError(f"Unsupported input type: {type(input_val)}")

if is_kwarg:

kwargs[name] = input_val

elif index is not None and index < len(args_list):

args_list[index] = input_val

else:

raise ValueError(

f"An unexpected error occurred while processing the input '{name}'. Please open an "

f"issue at https://github.com/huggingface/diffusers/issues and provide a minimal reproducible "

f"example along with the full stack trace."

)

return tuple(args_list), kwargs

def post_forward(self, module, output):

is_tensor = isinstance(output, torch.Tensor)

is_tensor_list = isinstance(output, (list, tuple)) and all(isinstance(x, torch.Tensor) for x in output)

if not is_tensor and not is_tensor_list:

raise ValueError(f"Expected output to be a tensor or a list/tuple of tensors, but got {type(output)}.")

output = [output] if is_tensor else list(output)

for index, cpm in self.metadata.items():

if not isinstance(cpm, ContextParallelInput) or not cpm.split_output:

continue

if index >= len(output):

raise ValueError(f"Index {index} out of bounds for output of length {len(output)}.")

current_output = output[index]

current_output = self._prepare_cp_input(current_output, cpm)

output[index] = current_output

return output[0] if is_tensor else tuple(output)

def _prepare_cp_input(self, x: torch.Tensor, cp_input: ContextParallelInput) -> torch.Tensor:

if cp_input.expected_dims is not None and x.dim() != cp_input.expected_dims:

logger.warning_once(

f"Expected input tensor to have {cp_input.expected_dims} dimensions, but got {x.dim()} dimensions, split will not be applied."

)

return x

else:

return EquipartitionSharder.shard(x, cp_input.split_dim, self.parallel_config._flattened_mesh)

class ContextParallelGatherHook(ModelHook):

def __init__(self, metadata: ContextParallelModelPlan, parallel_config: ContextParallelConfig) -> None:

super().__init__()

self.metadata = metadata

self.parallel_config = parallel_config

def post_forward(self, module, output):

is_tensor = isinstance(output, torch.Tensor)

if is_tensor:

output = [output]

elif not (isinstance(output, (list, tuple)) and all(isinstance(x, torch.Tensor) for x in output)):

raise ValueError(f"Expected output to be a tensor or a list/tuple of tensors, but got {type(output)}.")

output = list(output)

if len(output) != len(self.metadata):

raise ValueError(f"Expected output to have {len(self.metadata)} elements, but got {len(output)}.")

for i, cpm in enumerate(self.metadata):

if cpm is None:

continue

output[i] = EquipartitionSharder.unshard(output[i], cpm.gather_dim, self.parallel_config._flattened_mesh)

return output[0] if is_tensor else tuple(output)

class AllGatherFunction(torch.autograd.Function):

@staticmethod

def forward(ctx, tensor, dim, group):

ctx.dim = dim

ctx.group = group

ctx.world_size = torch.distributed.get_world_size(group)

ctx.rank = torch.distributed.get_rank(group)

return funcol.all_gather_tensor(tensor, dim, group=group)

@staticmethod

def backward(ctx, grad_output):

grad_chunks = torch.chunk(grad_output, ctx.world_size, dim=ctx.dim)

return grad_chunks[ctx.rank], None, None

class EquipartitionSharder:

@classmethod

def shard(cls, tensor: torch.Tensor, dim: int, mesh: torch.distributed.device_mesh.DeviceMesh) -> torch.Tensor:

# NOTE: the following assertion does not have to be true in general. We simply enforce it for now

# because the alternate case has not yet been tested/required for any model.

assert tensor.size()[dim] % mesh.size() == 0, (

"Tensor size along dimension to be sharded must be divisible by mesh size"

)

# The following is not fullgraph compatible with Dynamo (fails in DeviceMesh.get_rank)

# return tensor.chunk(mesh.size(), dim=dim)[mesh.get_rank()]

return tensor.chunk(mesh.size(), dim=dim)[torch.distributed.get_rank(mesh.get_group())]

@classmethod

def unshard(cls, tensor: torch.Tensor, dim: int, mesh: torch.distributed.device_mesh.DeviceMesh) -> torch.Tensor:

tensor = tensor.contiguous()

tensor = AllGatherFunction.apply(tensor, dim, mesh.get_group())

return tensor

def _get_submodule_by_name(model: torch.nn.Module, name: str) -> Union[torch.nn.Module, List[torch.nn.Module]]:

if name.count("*") > 1:

raise ValueError("Wildcard '*' can only be used once in the name")

return _find_submodule_by_name(model, name)

def _find_submodule_by_name(model: torch.nn.Module, name: str) -> Union[torch.nn.Module, List[torch.nn.Module]]:

if name == "":

return model

first_atom, remaining_name = name.split(".", 1) if "." in name else (name, "")

if first_atom == "*":

if not isinstance(model, torch.nn.ModuleList):

raise ValueError("Wildcard '*' can only be used with ModuleList")

submodules = []

for submodule in model:

subsubmodules = _find_submodule_by_name(submodule, remaining_name)

if not isinstance(subsubmodules, list):

subsubmodules = [subsubmodules]

submodules.extend(subsubmodules)

return submodules

else:

if hasattr(model, first_atom):

submodule = getattr(model, first_atom)

return _find_submodule_by_name(submodule, remaining_name)

else:

raise ValueError(f"'{first_atom}' is not a submodule of '{model.__class__.__name__}'")