[SPMD] Remove _specify_ddp_gpu_num method (#56425)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56425
As SPMD mode is gone, `_specify_ddp_gpu_num` becomes useless. It only checks if the module is a GPU module. This actually is already checked by the caller of this function (in fairscale and some other codebases).
Additionally also remove `enable_pytorch_sync_bn` wrapper that only calls this function and does nothing else.
ghstack-source-id: 126885376
Test Plan: waitforbuildbot
Reviewed By: zhaojuanmao
Differential Revision: D27866440
fbshipit-source-id: d2fd5cf43eda25c0a2bd35f647848ec0dbd6ad0f
