add output_past option to BERT class 
I need key-value of the present state like GPT class for BERT as well (I'm testing PPLM like architecture but with masked LM), and here I've added the option `output_past` to BERT to enable returning those statistics.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
