[DDP] Mark a few variables as const in reducer (#54764)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54764
We mark a few vars as const in Reducer, also do this for replicas_ and
process_group_ as they should not be changed by Reducer during training. This
can help eliminate issues at compile time and prevent the developer from
accidently changing these variables.
ghstack-source-id: 125040110
Test Plan: CI
Reviewed By: SciPioneer
Differential Revision: D27357132
fbshipit-source-id: 23a0edf754a8e4f9e6440e99860e5549724cb7ad
