Back out "[DDP][BE] Remove bucket replicas" (#73523)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73523
Original commit changeset: de8124e1628d
Original Phabricator Diff: D34399176 (https://github.com/pytorch/pytorch/commit/7fcf6c5c022268a8685e3aea9caf6648b8112549)
ghstack-source-id: 150128616
Test Plan: unit tests
Reviewed By: rohan-varma
Differential Revision: D34527919
fbshipit-source-id: 00914eeb6c84188b89d8fa8b32f8a4bf7c58774f
(cherry picked from commit e29aa43e36aa096e4a36de1fb7c477190a479cdf)
