[Reducer] Completely remove VariableIndex (#58592)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58592
Completely removes VariableIndex from reducer code, as it is not
needed. replica_index is always 0 so simplify the code to only use the
parameter index. Next, we should also remove all of the nested data structures
that were needed when num_replicas > 1 was possible.
ghstack-source-id: 129498226
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D28528440
fbshipit-source-id: e0568399264ab4f86de3b7a379a4f0831f8f42e9
