Save cnn model
    Returns
    ----------
    callback: A callback function that can be passed as epoch_end_callback to fit

def save_model():
    """Save cnn model
    Returns
    ----------
    callback: A callback function that can be passed as epoch_end_callback to fit
    """
    if not os.path.exists("checkpoint"):
        os.mkdir("checkpoint")
    return mx.callback.do_checkpoint("checkpoint/checkpoint", args.save_period)