fix: preserve method docstring through apply_forward_hook decorator

[ZAID646]

The @apply_forward_hook decorator wraps methods like AutoencoderKL.encode() and AutoencoderKL.decode() in a wrapper function without preserving the original method metadata. This causes the doc generator to miss these methods, making them absent from the online docs.

Using functools.wraps preserves __doc__, __name__, __module__, and __qualname__ through the decorator, ensuring the doc generator can discover these methods.

Fixes #6575