[FX] Add wrap() docstring to docs and add decorator example (#50555)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50555
Test Plan: Imported from OSS
Reviewed By: Chillee
Differential Revision: D25917564
Pulled By: jamesr66a
fbshipit-source-id: 20c7c8b1192fa80c6a0bb9e18910791bd7167232
