Make AutoProcessor a magic loading class for all modalities
# What does this PR do?

This PR re-enables a feature initially part of #14465 : the fact that `AutoProcessor` is a class loading the right processing class for any model (so processor, tokenizer or feature extractor). You can thus do:
```
processor = AutoProcessor.from_pretrained("bert-base-cased")
# Returns a fast BERT tokenizer
```
or
```
processor = AutoProcessor.from_pretrained("facebook/convnext-tiny-224")
# Returns a fConvNext feature extractor
```
_The documentation is not available anymore as the PR was closed or merged._