add a c++ interface in processGroup to get its backend name (#51066)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51066
backend name of a processgroup created using distributed_c10d python API is tracked, but there is no good way to track name of a processgroup created using processGroup c++ API. In some cases, knowing backend name of a processGroup is useful, e,g., log the backend name, or write some codes that have dependency on the known backend.
ghstack-source-id: 120628432
Test Plan: unit tests
Reviewed By: pritamdamania87
Differential Revision: D26059769
fbshipit-source-id: 6584c6695c5c3570137dc98c16e06cbe4b7f5503
