Document the default behavior for dist.new_group() when ranks=None (#44000)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44000
This wasn't documented, so add a doc saying all ranks are used when
ranks=None
ghstack-source-id: 111206308
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D23465034
fbshipit-source-id: 4c51f37ffcba3d58ffa5a0adcd5457e0c5676a5d
