Preserve HookRegistry forward signatures

[momomuchu]

Bug

HookRegistry.register_hook wrapped module.forward with a generic callable. After hook registration, inspect.signature(module.forward) reported:

(module, *args, **kwargs)

instead of the original model forward signature.

This broke torch.export keyword binding because kwargs such as hidden_states and timestep were bound against the wrapper signature.

Root cause

The replacement forward used functools.partial plus functools.update_wrapper, but it did not carry the original bound forward signature onto the installed wrapper.

Fix

Capture inspect.signature(forward) before wrapping, then assign it to self._module_ref.forward.__signature__ after installing the wrapped partial.

This keeps introspection aligned with the model API while leaving hook execution unchanged.

Tests

1. Added a regression test that a single hook registration preserves inspect.signature(model.forward).
2. Added a regression test that torch.export.export still accepts keyword inputs after hook registration.
3. Added regression tests for repeated instrumentation: after registering two hooks, inspect.signature(model.forward) still equals the original, and torch.export.export still binds keyword inputs. This covers the case where a second register_hook could otherwise capture the wrapper signature instead of the model signature.
4. Ran the relevant hooks suite:

DIFFUSERS_TEST_DEVICE=cpu python -m pytest -q tests/hooks/test_hooks.py

5. Ran targeted ruff and format checks on the changed files.

Fixes #14135.