DistributedDataParallel (DDP) is multi process training. For you case, you would get best performance with 8 DDP processes, where the i-th process calls:
torch.distributed.init_process_group(
    backend=‘nccl’, 
    init_method=‘tcp://localhost:1088’, 
    rank=i, 
    world_size=8
)

Or, you could set env vars and use https://github.com/pytorch/pytorch/blob/master/torch/distributed/launch.py