title: how to write a backward function for a caffe python layer with two top blobs?
body: <p>After installing caffe on ubuntu 16.04. I trained cifar10-quick model successfully, then I added a simple python layer as follow:</p>
  
  <pre><code>import caffe
  import numpy as np
  
  class My_Custom_Layer(caffe.Layer):  
      def setup(self, bottom, top):
          if len(bottom) != 1:
              raise Exception("Wrong number of bottom blobs")  
  
      def forward(self, bottom, top):
          top[0].data[...] = bottom[0].data
  
      def reshape(self, bottom, top):
          top[0].reshape(*bottom[0].shape)        
  
      def backward(self, top, propagate_down, bottom):
          bottom[0].diff[...] = top[0].diff
  </code></pre>
  
  <p>now I want to add another top blob, top[1] with some changes in bottom[0], but I have no idea how to change backward function!</p>