title: how to modify rnn cells in pytorch?
body: <p>If I want to change the compute rules in a RNN cell (e.g. GRU cell), what should I do?<br>
  I do not want to implement it via for or while loop considering the issue of efficiency.<br>
  I have viewed the source code of pytorch, but it seems that the major components of rnn cells are implement in c code which I cannot find and modify.
  You can answer this question through an example: implement GRU cell without the existing version.</p>
  
  <p>thank you ~</p>