Thanks a lot for your help! The code works well!
Since training_data is a list of zip object, it has to be unzipped first. The following code works:

for batch, batch_imgclass in enumerate(training_data):
    data = []
    label = []
    img, classLab = zip(*batch_imgclass)
    for image in img:
        image = image.float()
        data.append(image.unsqueeze(0))
    for true_label in classLab:
        true_label = true_label.float()
        label.append(true_label.unsqueeze(0))