[`Pix2Struct`] Fix pix2struct cross attention
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/25175

As pointed out by @leitro on the issue, I can confirm the cross-attention should be in `layer_outputs[5]`. Also fixes the attention output index that should be in index `3` as the index `2` is the `position_bias` (they have the same shape so we didn't noticed the silent bug on the CI tests.)

to repro:

```python
import requests
import torch
from PIL import Image
from transformers import Pix2StructForConditionalGeneration, Pix2StructProcessor

url = "https://www.ilankelman.org/stopsigns/australia.jpg"
image = Image.open(requests.get(url, stream=True).raw)

model = Pix2StructForConditionalGeneration.from_pretrained("google/pix2struct-textcaps-base")
processor = Pix2StructProcessor.from_pretrained("google/pix2struct-textcaps-base")

input_ids = torch.LongTensor([[0, 2, 3, 4]])

# image only
inputs = processor(images=image, return_tensors="pt")
outputs = model.forward(**inputs, decoder_input_ids=input_ids, output_attentions=True)

print(outputs.cross_attentions[0].shape)
>>> should be torch.Size([1, 12, 4, 2048])
```

cc @amyeroberts 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Fixed a slow test of torchscript that was failing, however the test:

```bash
tests/models/pix2struct/test_modeling_pix2struct.py::Pix2StructIntegrationTest::test_batched_inference_image_captioning_conditioned
```

is failing but can confirm is also failing on main, I think it is unrelated to this PR (env issues on my VM probably) so I am merging