I’m trying to run 2 gpu with torch tranning. 
from apex.parallel import DistributedDataParallel as DDP

   #multi gpu
    os.environ["CUDA_VISIBLE_DEVICES"] = '0, 1'
    
    torch.distributed.init_process_group(backend='nccl',
                                             init_method='env://')
    #model = nn.DataParallel(model, output_device=1)
    model = DDP(model, delay_allreduce=True)

I added this init_process_group and DDP part 

this is not running without error 

and I dont know what backend and init_method mean