Update `past_key_values` in GPT-2 (#9596)
* Update past_key_values in gpt2 (#9391)
* Update generation_utils, and rename some items
* Update modeling_gpt2 to avoid an error in gradient_checkpointing
* Remove 'reorder_cache' from util and add variations to XLNet, TransfoXL, GPT-2
* Change the location of '_reorder_cache' in modeling files
* Add '_reorder_cache' in modeling_ctrl
* Fix a bug of my last commit in CTRL
* Add '_reorder_cache' to GPT2DoubleHeadsModel
* Manage 'use_cache' in config of test_modeling_gpt2
* Clean up the doc string
* Update src/transformers/models/gpt2/modeling_gpt2.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* Fix the doc string (GPT-2, CTRL)
* improve gradient_checkpointing_behavior
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
