[SPMD] Remove ddp_gpu_size field from SyncBatchNorm (#55946)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55946
As `ddp_gpu_size` field of `SyncBatchNorm` will always be 1 for GPU modules, remove this field and the relevant code.
ghstack-source-id: 126883498
Test Plan: waitforbuildbot
Reviewed By: zhaojuanmao
Differential Revision: D27746021
fbshipit-source-id: b4518c07e6f0c6943fbd7a7548500a7d4337126c
