text: Title: Question about the loss of Masked LM
  Body: Thank you very much for this great contribution. 
  I found the loss of masked LM didn't decrease when it reaches the value around 7. However, in the official  tensorflow implementation, the loss of MLM decreases to 1 easily. I think something went wrong in your implementation. 
  In additional, I found the code can not predict the next sentence correctly. I think the reason is: `self.criterion = nn.NLLLoss(ignore_index=0)`.   It can not be used as criterion for sentence prediction because the label of sentence is 1 or 0. We should remove ignore_index=0 for sentence prediction. 
  I am looking forward to your reply~