code: def get_image_grad(net, image, class_id=None):
      
      return _get_grad(net, image, class_id, image_grad=True)
docs: Get the gradients of the image.
  
      Parameters:
      ----------
      net: Block
          Network to use for visualization.
      image: NDArray
          Preprocessed image to use for visualization.
      class_id: int
          Category ID this image belongs to. If not provided,
          network's prediction will be used.