Add docstring for Proxy (#50145)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50145
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D25854281
Pulled By: ansley
fbshipit-source-id: d7af6fd6747728ef04e86fbcdeb87cb0508e1fd8
