Add a regression test for force_accelerate_hooks signature preservation (#48260)

* Add a regression test for force_accelerate_hooks signature preservation

Covers what the missing `functools.wraps` broke: `inspect.signature()` on a
method carrying the decorator, binding captured inputs against it, and the
`__name__` / `__doc__` metadata. Removing the `functools.wraps` line makes it
fail with

    AssertionError: Lists differ: ['args', 'kwargs'] != ['hidden_states', ...]

* update tests

* update comments for reference

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Co-authored-by: Marc Sun <57196510+SunMarc@users.noreply.github.com>