Revert D30888794: [Model Averaging] Simplify PostLocalSGD Optimizer API
Test Plan: revert-hammer
Differential Revision:
D30888794 (https://github.com/pytorch/pytorch/commit/3d312b3b8ee90f8b289c7d5601a13d0521b46b7e)
Original commit changeset: 21261b480f6b
fbshipit-source-id: 87abb7e8cd9ecaac909ec6c3ee053fa7c4ae1975
