Mistral qna
Author: sriram-1
Pull request: yes

# What does this PR do?
Added a MistralForQuestionAnswering class to the [modeling_mistral.py](https://github.com/huggingface/transformers/blob/main/src/transformers/models/mistral/modeling_mistral.py) so Mistral models have AutoModelForQuestionAnswering support.
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Fixes #28908

## 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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## Who can review?
@ArthurZucker
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Comments:
@ArthurZucker https://github.com/huggingface/transformers/issues/28908 already feature request is tagged. 

Then let's wait until:
- either community pushes for this
- there are good checkpoints for this model and task! 🥳 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.