[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

docs/source/en/model_doc/pix2struct.mdx

@@ -28,9 +28,8 @@ We therefore advise you to use these models for the tasks they have been fine tu
 This model was contributed by [ybelkada](https://huggingface.co/ybelkada).
 The original code can be found [here](https://github.com/google-research/pix2struct).
 
-## Resources:
+## Resources
 
-- [Paper](https://arxiv.org/abs/2210.03347)
 - [Fine-tuning Notebook](https://github.com/huggingface/notebooks/blob/main/examples/image_captioning_pix2struct.ipynb)
 - [All models](https://huggingface.co/models?search=pix2struct)
 
@@ -70,4 +69,4 @@ The original code can be found [here](https://github.com/google-research/pix2str
 ## Pix2StructForConditionalGeneration
 
 [[autodoc]] Pix2StructForConditionalGeneration
-    - forward
+    - forward