[javierdejesusda]

What does this PR do?

Fixes #6575

apply_forward_hook returns an inner wrapper that doesn't use functools.wraps. When accelerate 0.17 or newer is installed, every method it decorates (37 files in src/diffusers/ use it, mostly on encode/decode) loses its __name__ and __doc__, has no __wrapped__, and reports the signature (self, *args, **kwargs). inspect.signature, help() and doc-builder all see wrapper, so with accelerate installed doc-builder's autodoc renders AutoencoderKL.encode/decode as wrapper(*args, **kwargs), with no docstring and a source link to accelerate_utils.py.

The hosted v0.39.0 docs still show them that way. The v0.40.0 and main docs render them correctly because #14131 moved the doc build to doc-builder's light install, which doesn't install accelerate, so the decorator returns the method unchanged there. apply_forward_hook on main still doesn't use functools.wraps.

This adds @functools.wraps(method) to wrapper. An accelerate pre_forward hook still fires on encode/decode calls, same as on main.

With accelerate installed:

# main
AutoencoderKL.decode.__name__                 == "wrapper"
AutoencoderKL.decode.__doc__                  is None
hasattr(AutoencoderKL.decode, "__wrapped__")  == False
inspect.signature(AutoencoderKL.decode)       == (self, *args, **kwargs)

# this PR
AutoencoderKL.decode.__name__                 == "decode"
AutoencoderKL.decode.__doc__                  starts with "Decode a batch of images."
hasattr(AutoencoderKL.decode, "__wrapped__")  == True
inspect.signature(AutoencoderKL.decode)       == (self, z: torch.FloatTensor, return_dict: bool = True, generator=None) -> DecoderOutput | torch.FloatTensor

Coordination: in #6575, stevhliu linked the @apply_forward_hook line above AutoencoderKL.decode and wrote "I think the line here may be giving the doc-builder some hiccups when parsing the file, but I'm not entirely sure." On this PR, his review said "nice, thanks for this!" and left the call on whether to keep the test to sayakpaul.

Tests

TestApplyForwardHook in tests/others/test_utils.py checks that a decorated function keeps its __name__ and __doc__. It only catches a missing functools.wraps when accelerate is installed: without accelerate, apply_forward_hook returns the function unchanged and the test passes with or without the fix. The "Fast tests for PRs" workflow installs accelerate and runs tests/others.

Run with Python 3.11, torch 2.8.0 and accelerate 1.15.0:

$ pytest tests/others/test_utils.py::TestApplyForwardHook    # main (916a5791e) with only this test added
E       AssertionError: assert 'wrapper' == 'example'
1 failed

$ pytest tests/others/test_utils.py::TestApplyForwardHook    # this branch
1 passed

$ pytest tests/others/test_utils.py
25 passed, 1 skipped

$ pytest -n 4 tests/models/autoencoders/test_models_autoencoder_kl.py tests/models/autoencoders/test_models_consistency_decoder_vae.py tests/models/autoencoders/test_models_vq.py tests/models/autoencoders/test_models_autoencoder_tiny.py tests/models/autoencoders/test_models_asymmetric_autoencoder_kl.py tests/hooks/test_hooks.py tests/hooks/test_group_offloading.py
113 passed, 206 skipped    # same counts on main

$ make quality
All checks passed!
2138 files already formatted

Claude was used to review this change.

Who can review?

@stevhliu @sayakpaul

[HuggingFaceDocBuilderDev]

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[stevhliu]

nice, thanks for this! imo seems a bit much to add a test for this simple fix, but i'll defer to @sayakpaul on whether we need to include this

[javierdejesusda]

Happy to defer on the test. I only added it because the #6575 breakage was silent: call-time behaviour was always fine, it only showed up in inspect.signature/doc-builder, so a functional test wouldn't catch a regression if functools.wraps gets dropped later. That said, it's a 2-line fix, so I'm happy to keep it as-is, trim it to a single minimal assertion, or drop it entirely, whatever you and @sayakpaul prefer.

[javierdejesusda]

@sayakpaul I've updated the branch with main and trimmed the test down to a single minimal check (the wrapped method keeps its __name__ and __doc__), as @stevhliu suggested. Ready for review whenever you have a moment.

[javierdejesusda]

@sayakpaul I merged main again to fix the conflict in tests/others/test_utils.py (#14792 added a test class in the same spot, so I kept both) and corrected the description. The test is still the trimmed single check from before, after stevhliu's note that a test seemed a bit much for this fix. Happy to drop it if you'd prefer.