Add MaskedImageModelingOutput 
# What does this PR do?
- Adds `MaskedImageModelingOutput` and `TFMaskedImageModelingOutput` classes for masked image modeling / completion / in-painting models.
- Replaces the inaccurate MaskedLMOutput used for ViT and DeiT MIM heads with the new output class
- Ensures backward compatibility by adding `logits` as a property to the new output class

## Before submitting
- [X ] 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.
- [ 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).
- [ ] Did you write any new necessary tests?



_The documentation is not available anymore as the PR was closed or merged._<|||||>Pinging @NielsRogge for the final approval<|||||>@NielsRogge could you take another look? I think all comments are addressed