Move the model type check in DocumentQuestionAnswering to support Donut

[ankrgyl]

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).
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• Did you write any new necessary tests?

Who can review?

@NielsRogge @Narsil

[NielsRogge]

Thanks for fixing, looks clean to me!

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

LGTM, thanks for the fix!