[Model Averaging] Allow subgroup to be None in PostLocalSGDState (#63277)

[Model Averaging] Allow subgroup to be None in PostLocalSGDState (#63277)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63277

`PostLocalSGDState` requires a subgroup. To initialize this subgroup, a global process group must be initialized. However, this imposes a restriction that a hook state can only be provided after distributed environment initialization, which is not compatible with lightning DDP plugin setup where hook state should be provided before distributed environment initialization.

Proposal: https://github.com/pytorch/pytorch/issues/59699
ghstack-source-id: 135848575

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

Reviewed By: cbalioglu

Differential Revision: D30325041

fbshipit-source-id: 7b870166d096d306c3f2f7c69816a705cec0bebd