Fix processors
# What does this PR do?

This PR aims to fix import checks for processors of multi-modal models, like those of LayoutLMv2, TrOCR and ViLT.

The bare minimum for a processor is PIL (assuming one creates a processor by combining a feature extractor and a slow tokenizer). If one combines a feature extractor and a fast tokenizer, both PIL and tokenizers should be available.

Questions:
- should I also update the general init file?
- should I add the availability to load both the slow and fast tokenizer to a processor? e.g. ViltProcessor currently only allows the fast one

To do:

- [ ] Fix AutoProcessor API for TrOCR (#14884) 
- [ ] Fix the slow integration tests (by not using the `AutoTokenizer` API in the `from_pretrained` method)
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15280). All of your documentation changes will be reflected on that endpoint.<|||||>A much better fix was provided in #15549. Therefore, closing.