Put `learn` on distributed training with `cuda_id`.

def _learner_distributed(learn:Learner, cuda_id:int, cache_dir:PathOrStr='tmp'):
    "Put `learn` on distributed training with `cuda_id`."
    learn.callbacks.append(DistributedTrainer(learn, cuda_id))
    learn.callbacks.append(DistributedRecorder(learn, cuda_id, cache_dir))
    return learn