Add deprecation warning to PG backend and make TP backend stable. (#45356)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45356
In this PR, I'm adding a warning to the PG backend mentioning it would
be deprecated in the future. In addition to this I removed the warning from the
TP backend that it is a beta feature.
ghstack-source-id: 112940501
Test Plan: waitforbuildbot
Reviewed By: mrshenli
Differential Revision: D23940144
fbshipit-source-id: d44054aa1e4ef61004a40bbe0ec45ff07829aad4
