[DDP] remove dedupe check in reducer (#53919)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53919
https://github.com/pytorch/pytorch/pull/53279/files has landed
deduplicating the shared params in python before constructing reducer. Because
of this, we no longer need the changes in
https://github.com/pytorch/pytorch/pull/46755/files.
This is already tested by `test_ddp_shared_grad_acc_unused_params` and
`test_ddp_weight_sharing`
ghstack-source-id: 123828299
Test Plan: ci
Reviewed By: SciPioneer
Differential Revision: D27015466
fbshipit-source-id: efb079540c1a0e18bb38e68479caeb50cf550304
