[DDP Note] Remove refs to RoundRobin PG until we officially support it (#40380)
Summary:
Removes line mentioning `ProcessGroupRoundRobin` since we don't intend it to be used as a public API just yet. We can add this back when we officially support the API
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40380
Differential Revision: D22165556
Pulled By: rohan-varma
fbshipit-source-id: 24d0477d881dc74f2ff579de61dfd1ced2b09e75
