[Model Averaging] Add a few member methods of PostLocalSGDOptimizer (#63340)

[Model Averaging] Add a few member methods of PostLocalSGDOptimizer (#63340)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63340

Some methods are needed such as accessing optimizer states. These are necessary for integration with PyTorch Lightning.

Proposal: https://github.com/pytorch/pytorch/issues/59699
ghstack-source-id: 135912246

Test Plan: buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork -- test_ddp_hook_parity_post_localSGD

Reviewed By: rohan-varma

Differential Revision: D30328794

fbshipit-source-id: e585b874313bd266fdc7c79936e2af98700c7bad