Add assistant prefill for chat templates and TextGenerationPipeline (#33198)
* Add assistant prefill to chat templates
* Add assistant prefill to pipeline
* Add assistant prefill to pipeline
* Tweak another test that ended in assistant message
* Update tests that ended in assistant messages
* Update tests that ended in assistant messages
* Replace assistant_prefill with continue_final_message
* Allow passing continue_final_message to pipeline
* Small fixup
* Add continue_final_message as a pipeline kwarg
* Update docstrings
* Move repos to hf-internal-testing!
* Update src/transformers/tokenization_utils_base.py
Co-authored-by: Lysandre Debut <hi@lysand.re>
* Add explanatory comment
* make fixup
* Update chat templating docs to explain continue_last_message
---------
Co-authored-by: Lysandre Debut <hi@lysand.re>