seems something to do with the  pin_memory ,what do this argument mean ?
val_loader = torch.utils.data.DataLoader(
        MyImageFolder(valdir, transforms.Compose([
            transforms.Scale(256),
            transforms.CenterCrop(224),
            transforms.ToTensor(),
            normalize,
        ])),
      batch_size=1, shuffle=False,
        num_workers=4, pin_memory=False)
        #num_workers=4, pin_memory=True)

this works