Create MaskedImageCompletionOutput and fix ViT docs
# What does this PR do?
Fixes the output class and docs of `ViTForMaskedImageModeling `which returns images of shape (batch_size, num_channels, height, width) whereas MaskedLMOutput logits are of shape (batch_size, seq_length, vocab_size).

**Notes:**
- We have no checkpoints for `ViTForMaskedImageModeling` and the docstrings use the pretrained ViTModel combined with random head parameters. I will open a separate PR for the other affected model - `DeiTForMaskedImageModeling`, for which no checkpoints are available either. 
- Swin has its own MaskedImageOutput class but only has base model checkpoints (trained on the masked image modeling task) and no task-specific checkpoints.
- I'm planning to create a masked-image-completion pipeline and add Swin and ICT (once Sheon's PR is merged).

CC: @sheonhan

## Before submitting
- [X ] 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?
- [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._<|||||>Gently pinging @amyeroberts for the final approval<|||||>masked-image-completion pipeline sounds awesome 🙌