investigate how to pass past_key_values to generate()

[jemmyshin]

We already implemented passing past_key_values to step_generate() to enable cache in LLM.

generate() also support this but should have a different method to implement this since generate() will truncate the input_ids if past_key_values is provided.

[jemmyshin]

conclusion for now:

We can pass past_key_values to generate(), however, it's not possible to return the updated past_key_values from generate(). We can modify the utils in huggingface package to enable this but it's a hacky way, so we decide now to implement this feature now.