Hi, thanks for reply. No, no errors.  I understand what you mean. The problem for me is at this step for generating the torchvision.datasets.Kinetics400 instance (before feed it to the dataloader), it took too long and exceed the time limit for my computing cluster. But I just realized I can add num_workers, like
dataset = torchvision.datasets.Kinetics400(
            traindir,
            frames_per_clip=250,
            step_between_clips=200,
            transform=transform_train,
            frame_rate=30,
            extensions=('avi', 'mp4', ),
            num_workers=4
        )

And it seems to be able to run smoothly, so I am testing if it helps.
But still wonder if it make sense to add the num_workers argument here.