[NCCL] support send/recv to/from self when communicator is created on demand (#45873)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45873
This diff adds support for sending/receiving to/from self. It also fixed a bug when p2p operations are not used by all processes.
ghstack-source-id: 113910526
Test Plan: waitforsandcastle
Reviewed By: jiayisuse
Differential Revision: D24124413
fbshipit-source-id: edccb830757ac64f569e7908fec8cb2b43cd098d
