Fix pix2struct
Author: IlyasMoutawwakil
Pull request: yes

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In #34089 some stuff might have not being copied correctly from T5. In pix2struct case past_key_value was being overridden by the first attention block, resulting in failure in cross attention.

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Comments:
In transformers v4.46 generation for Pix2Struct fail when setting `use_cache=True`


Code to reproduce this issue : https://huggingface.co/docs/transformers/v4.46.0/en/model_doc/pix2struct#transformers.Pix2StructForConditionalGeneration.forward.example with `use_cache=True`
```
from PIL import Image
import requests
from transformers import AutoProcessor, Pix2StructForConditionalGeneration
model_id = "google/pix2struct-textcaps-base"

processor = AutoProcessor.from_pretrained(model_id)
model = Pix2StructForConditionalGeneration.from_pretrained(model_id)
url = "https://www.ilankelman.org/stopsigns/australia.jpg"
image = Image.open(requests.get(url, stream=True).raw)
inputs = processor(images=image, return_tensors="pt")
generated_ids = model.generate(**inputs, max_new_tokens=50, use_cache=True)
```

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_34374). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@ArthurZucker I added a small quick test

there's one thing I don't understand, running the same test with T5 passes even without eval mode, but it was necessary to get it passing for pix2struct.