


creating process group (optional)
     >>> # ranks is a list of int identifying rank ids.
     >>> ranks = list(range(8))
     >>> r1, r2 = ranks[:4], ranks[4:]
     >>> # Note: every rank calls into new_group for every
     >>> # process group created, even if that rank is not
     >>> # part of the group.
     >>> process_groups = [torch.distributed.new_group(pids) for pids in [r1, r2]]
     >>> process_group = process_groups[0 if dist.get_rank() <= 3 else 1]
     >>> sync_bn_module = torch.nn.SyncBatchNorm.convert_sync_batchnorm(module, process_group)



