Supporting create an RPC gang of size 1 (#32731)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32731
As we now support send to self, we no longer require world_size > 1.
Removing the assert from ProcessGroupAgent.
Test Plan: Imported from OSS
Differential Revision: D19609558
Pulled By: mrshenli
fbshipit-source-id: ecec18d756f97d8d78d4526a63b7cb8ab6f858a3
