[FX] Fix using fx.wrap as a decorator (#50677)
Summary:
`torch.fx.wrap()` could not be used as a decorator as the docstring claimed because it returned None.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50677
Test Plan: Added `test_wrapped_via_decorator` which used to fail with `'NoneType' object is not callable` and now passes
Reviewed By: jamesr66a
Differential Revision: D25949313
Pulled By: jansel
fbshipit-source-id: 02d0f9adeed812f58ec94c94dd4adc43578f21ce
