text: Title: Question about the backward of the quantize function
  Body: Usage Questions Only:
  I have a question about quantize function in the dorefa paper.
  
  ![_20180118155814](https://user-images.githubusercontent.com/19725474/35086679-6ea44784-fc68-11e7-981d-03cd69819feb.png)
  
  I  have confused to the process about find out the derivative r0/ri
  
  
  