Add DebertaV2ForMultipleChoice
Author: zphang
Pull request: yes

# What does this PR do?

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Adds `DebertaV2ForMultipleChoice`

## 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?

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@LysandreJik

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Models:

- albert, bert, xlm: @LysandreJik
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Documentation: @sgugger

HF projects:

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Also for additional context, the implementation of the multiple choice model is taken from `RobertaForMultipleChoice`.

Thanks for your contribution @zphang!