[Opt Overlap] Implement _OverlappedOptimizer (#71605)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71605
ghstack-source-id: 147577173
Test Plan: CI
Reviewed By: cbalioglu
Differential Revision: D33692686
fbshipit-source-id: b0fdb45245d923e1de8fef4431d3e235ac57dcbf
(cherry picked from commit 8b83dbf690d6f426cc5f3954d77c829a3d302962)
