def scale_set(image_set):
    """
    Given a batch of images, normalizes each image by converting pixels to [-1, +1]
    image_set: (batch_size, C, H, W)
    returns: scaled image_set (batch_size, C, H, W)
    """
    # Global means of imagenet channels [123.68, 116.779, 103.939]
    return 2. * ((image_set / 255.) - 0.5)