As far as I am aware, there is no faster way to accomplish this task.

  
    
    
    How to speed up for loop in customized RNN? 
  
  
    no unfortunately it is not a problem with the loop.  I run the cell 1000 times and it take 4.6 sec in total. Looking at the details of each run, each of them takes in average, 0.0045 sec. So the loop account for very little. Thanks though!