Make chat templates part of ProcessorMixin (#30744)
* Let's try moving chat templates out of IDEFICS and into the generic ProcessorMixin
* Chat templates should not be mandatory
* Chat templates should not be mandatory
* Not all classes will have default chat templates
* stash commit
* Add chat template docstring
* Clean up docstring
* Add chat templates to LLaVA/LLaVA-next
* Docstring fixup
* Quick IDEFICS2 fixup
* Remove some old references to the Conversation class
* make fixup
