Add Image To Text Generation pipeline
# What does this PR do?

Add Image To Text Generation pipeline. The pipeline currently defaults to [nlpconnect/vit-gpt2-image-captioning](https://huggingface.co/nlpconnect/vit-gpt2-image-captioning).

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


## Who can review?

This features was asked for by @Narsil.

_The documentation is not available anymore as the PR was closed or merged._<|||||>cc @mishig25, the inference widgets for models like TrOCR, Donut, [image captioning](https://huggingface.co/nlpconnect/vit-gpt2-image-captioning) can now be created! 🥳 