The usage of past_key_values in AutoCompressorMixin

[RewindL]

Hello authors. There is no doubt that AutoCompressors is an excellent work.

I have carefully read and learned the mainly designed codes in auto_compressor.py and noticed that the class AutoCompressorMixin contains quite a bit of codes for processing past_key_values (including softprompt in it). But i checked the intermediate values of ​​past_key_values in forward() during training and found it seems always be None.

That confuses me and I have question that whether the processing of past_key_values ​​is redundant in your code for aligning standard interfaces for CasualLM , or for other possible purposes？If not, I'm curious about what the situation is past_key_values!=None during model forwarding ?