add documentation to shape inference algorithm (#64312)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/64312
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D30709254
Pulled By: migeed-z
fbshipit-source-id: 3297d26fe6727c5b9ca176625b1683d787f59659
