code: def input_diff(orig_img):
    
    def inner(T):
      diff = T("input") - orig_img
      return tf.sqrt(tf.reduce_mean(diff**2))
    return inner
docs: Average L2 difference between optimized image and orig_img.
  
    This objective is usually mutliplied by a negative number and used as a
    penalty in making advarsarial counterexamples.