Move the model type check in DocumentQuestionAnswering to support Donut
# What does this PR do?

Prior to this change, you'd see an error while instantiating a pipeline with Donut:

```
In [3]: pipe = pipeline(task="document-question-answering", model='naver-clova-ix/donut-base-finetuned-docvqa')
The model 'VisionEncoderDecoderModel' is not supported for document-question-answering. Supported models are ['LayoutLMForQuestionAnswering', 'LayoutLMv2ForQuestionAnswering', 'LayoutLMv3ForQuestionAnswering'].
```

because it's not part of `AutoModelForDocumentQuestionAnswering`. I've moved around that check so that it does not apply to the Donut case.

Fixes #18926


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@NielsRogge @Narsil
_The documentation is not available anymore as the PR was closed or merged._