Add documentation page for pipeline parallelism. (#50791)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50791
Add a dedicated pipeline parallelism doc page explaining the APIs and
the overall value of the module.
ghstack-source-id: 120257168
Test Plan:
1) View locally
2) waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D25967981
fbshipit-source-id: b607b788703173a5fa4e3526471140506171632b
