Enhance `new_group` doc to mention using NCCL concurrently. (#48872)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48872
Using NCCL communicators concurrently is not safe and this is
documented in NCCL docs.
However, this is not documented in PyTorch and we should add documentation for
ProcessGroupNCCL so that users are aware of this limitation.
ghstack-source-id: 118148014
Test Plan: waitforbuildbot
Reviewed By: rohan-varma
Differential Revision: D25351778
fbshipit-source-id: f7f448dc834c47cc1244f821362f5437dd17ce77
