text: Title: A question about loss backward
  Body: Thank you for sharing the well organized code. I learned a lot from the code. Now I would like to ask you a question about loss.backward. For the classification problem, the final output of the model is a classification vector, such as 1 x M vector, your code is also targeted at the classification task, for a certain category, one of the categories corresponding to the classification score can be propagated backward. My question is, if the output of the model is not a 1 x M vector, but a matrix like N x M, and I want to take one of N and propagate backwards, how do I do that? 
  
     Your code: 1 x M -> value
     I want: N x M -> a vector
  
  Looking forward to your reply.