Add ViltForTokenClassification e.g. for Named-Entity-Recognition (NER)
# What does this PR do?

Adding ViltForTokenClassification in order to be able to fine-tune ViLT for a token classification task (e.g. as Named-Entity-Recognition). This allows leveraging both image and text for token classification tasks using ViLT. 

## 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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      Pull Request section?
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- [X] 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).
- [X] Did you write any new necessary tests?


## Who can review?

@xhluca , @LysandreJik, @NielsRogge, @ydshieh 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @gilad19, the implementation and internals look good to me! 

I'll defer to @NielsRogge regarding the forward call implementation and documentation.<|||||>Regarding the use case - exactly, apply NER on a piece of text for which you also have a visual information. <|||||>Hi @NielsRogge - a gentle reminder :) <|||||>Feel free to merge when satisfied @NielsRogge 