src/transformers/training_args.py/TrainingArguments/_setup_devices
class TrainingArguments:    def _setup_devices(self) -> "torch.device":
        logger.info("PyTorch: setting up devices")
        if self.no_cuda:
            device = torch.device("cpu")
            self._n_gpu = 0
        elif is_torch_tpu_available():
            device = xm.xla_device()
            self._n_gpu = 0
        elif is_sagemaker_mp_enabled():
            local_rank = smp.local_rank()
            device = torch.device("cuda", local_rank)
            self._n_gpu = 1
        elif is_sagemaker_dp_enabled():
            sm_dist.init_process_group()
            self.local_rank = sm_dist.get_local_rank()
            device = torch.device("cuda", self.local_rank)
            self._n_gpu = 1
        elif self.deepspeed:
            # deepspeed inits torch.distributed internally
            from .deepspeed import is_deepspeed_available

            if not is_deepspeed_available():
                raise ImportError("--deepspeed requires deepspeed: `pip install deepspeed`.")
            import deepspeed

            deepspeed.init_distributed()

            # workaround for setups like notebooks where the launcher can't be used,
            # but deepspeed requires a dist env.
            # env LOCAL_RANK could be set manually by the user, or via init_distributed if mpi4py is installed
            self.local_rank = int(os.environ.get("LOCAL_RANK", "-1"))

            device = torch.device("cuda", self.local_rank)
            self._n_gpu = 1
        elif self.local_rank == -1:
            # if n_gpu is > 1 we'll use nn.DataParallel.
            # If you only want to use a specific subset of GPUs use `CUDA_VISIBLE_DEVICES=0`
            # Explicitly set CUDA to the first (index 0) CUDA device, otherwise `set_device` will
            # trigger an error that a device index is missing. Index 0 takes into account the
            # GPUs available in the environment, so `CUDA_VISIBLE_DEVICES=1,2` with `cuda:0`
            # will use the first GPU in that env, i.e. GPU#1
            device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
            # Sometimes the line in the postinit has not been run before we end up here, so just checking we're not at
            # the default value.
            self._n_gpu = torch.cuda.device_count()
        else:
            # Here, we'll use torch.distributed.
            # Initializes the distributed backend which will take care of synchronizing nodes/GPUs
            torch.distributed.init_process_group(backend="nccl")
            device = torch.device("cuda", self.local_rank)
            self._n_gpu = 1

        if device.type == "cuda":
            torch.cuda.set_device(device)

        return device