[Pix2struct] Simplify generation (#22527)
* Add model to doc tests
* Remove generate and replace by prepare_inputs_for_generation
* More fixes
* Remove print statements
* Update integration tests
* Fix generate
* Remove model from auto mapping
* Use auto processor
* Fix integration tests
* Fix test
* Add inference code snippet
* Remove is_encoder_decoder
* Update docs
* Remove notebook link
