Cleanup APIs for pipeline parallelism. (#48630)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48630
1) Make torch.distributed.pipeline package public.
2) Make several helper methods private.
ghstack-source-id: 118820803
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D25235688
fbshipit-source-id: c32833ebf090ddbd4eaf06fcb5e3f9d421623a60
