Add documentation for lowering (#71116)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71116
As title, add more inline documentation for code.
Test Plan:
no
pingpoke
Reviewed By: 842974287
Differential Revision: D33465611
fbshipit-source-id: 6b5529893098e5591470c2f41a0d8989e3cfccb9
