[SPMD] Error out SPMD in C++ Reducer (#55212)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55212
Error out SPMD in C++ Reducer.
Added a new test `test_reducer_no_multi_replicas`, which checks no multiple replicas are allowed at the Reducer constructor.
Removed 2 tests relevant to reducer in SPMD mode:
`test_ddp_comm_hook_multiple_replica_check`
`test_forward_backward_multi_replica`
ghstack-source-id: 125602472
Test Plan: waitforbuildbot
Reviewed By: pritamdamania87
Differential Revision: D27497747
fbshipit-source-id: 17ef1bc4d889cbe8076bcb3d504aed4c1aea1562
