Improve documentation for pipeline parallelism. (#48638)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48638
Polishing up some of the docs for the main `Pipe` class and its
`forward` method.
ghstack-source-id: 118820804
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D25237705
fbshipit-source-id: ba3d8737b90a80024c827c0887fc56f14bf678b7
