[Pix2struct] Simplify generation
# What does this PR do?

This PR aims to fix the warning that is currently printed out when generating text with Pix2Struct:

```
A decoder-only architecture is being used, but right-padding was detected! For correct generation results, please set `padding_side='left'` when initializing the tokenizer.
```
I see that all Pix2Struct models have `config.is_encoder_decoder=False`, but as Pix2Struct is an encoder-decoder model it'd be great/more logical to have this argument set to `True` and instead overwrite `prepare_inputs_for_generation` to have a cleaner way of generating text. This also makes us get rid of the warning.

To do:

- [ ] for the moment there're still one integration test failing (`test_batched_inference_image_captioning_conditioned`):

```
AssertionError: 'An photography of the Temple Bar and a collection of other items.' != 'An photography of the Temple Bar and a few other places.'
E       - An photography of the Temple Bar and a collection of other items.
E       ?                                        ^^^^ ^^^^^^^^       ^^ -
E       + An photography of the Temple Bar and a few other places.
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>PR is ready for review, however checkpoints on the hub will need to be updated (`is_encoder_decoder` = True) for this PR to be merged<|||||>PR is ready, models on the hub don't need to be updated since they don't have `is_encoder_decoder` set on the model config level (i.e. `Pix2StructConfig`. They have set it only in `Pix2StructTextConfig`). cc @younesbelkada 