[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

utils/documentation_tests.txt

@@ -306,6 +306,7 @@ src/transformers/models/pegasus/tokenization_pegasus.py
 src/transformers/models/pegasus/tokenization_pegasus_fast.py
 src/transformers/models/perceiver/tokenization_perceiver.py
 src/transformers/models/phobert/tokenization_phobert.py
+src/transformers/models/pix2struct/modeling_pix2struct.py
 src/transformers/models/plbart/tokenization_plbart.py
 src/transformers/models/prophetnet/tokenization_prophetnet.py
 src/transformers/models/rag/tokenization_rag.py