@mrshenli @albanD. I found this  SyncBatchNorm — PyTorch 1.7.0 documentation 9  . seems normal BN can not be used with DDP.  but I dont know how to use this in my code.  in specific I can not understand this line.
process_group = torch.distributed.new_group(process_ids)

I already initialised a process group using the:
dist.init_process_group(backend='nccl',init_method='env://',world_size=args.num_gpus ,rank=rank)    

why should I make a new_group() and how can I get process_ids