Auto processor
# What does this PR do?

This PR adds an `AutoProcessor` API, similar to `AutoTokenizer` and `AutoFeatureExtractor`.
I can amend the PR to have the auto-processor then go to a tokenizer (if available) or a feature extractor (if available), as I think that's the logic we want anyway.<|||||>Discussed it a bit with @patrickvonplaten that isn't as excited as I am about the `AutoProcessor` being an umbrella over all modalities' preprocessors and he raises important API questions.

Will let him comment so that we all align on the choices :) <|||||>I see the need for a `AutoProcessor` class, but I'm not a fan of making it an umbrella class for both tokenizers, feature extractors and processors because:

i) It goes a bit against our "no-magic" & easy-to-understand code IMO. Having `AutoProcessor` wrap both `AutoTokenizer` makes this code quite difficult to understand. E.g. if for some reason this class fails to load an NLP tokenizer, the traceback can become quite complex (`AutoProcessor` -> `AutoTokenizer` -> (here multiple ways of loading the `AutoTokenizer` via `tokenizer_config`, `tokenizer_type`, model `config`). Also I'm quite sure this function will become much more complex over time to handle all kinds of weird use cases. We could limit this complexity by not making it return `AutoFeatureExtractor` or `AutoTokenizer`

ii) IMO it breaks a design pattern. So far we had the following design pattern IMO:
- AutoTokenizer returns a tokenizer of type  `PreTrainedTokenizer` and `PretrainedTokenizerFast` 
- AutoFeatureExtractor returns a feature extractor of type `FeatureExtractionMixin`.
-> both of those classes has IMO more or less the same design.
It is much more intuitive IMO that `AutoProcessor` only return `...Processor` objects and nothing more. Also, I understand a `...Processor` not really as a general "whatever-you-can-process" class, but as a wrapper object that **always** includes two or more pre- or postprocessing objects (e.g. a speech input pre-processor and text output post-processor). Admittingly the naming is not great here though as `...Processor` does encompass pretty much all kinds of tokenization, feature extraction, etc...

iii) I don't see the use-case of this class really. IMO there is no need to force an `Auto...` class to be useful for more than one task (or modality). E.g. I don't think many users are keen to have a single script in which they can quickly switch between an text tokenizer and a speech recognition processor => for me the beauty of `Auto...` is to be able to quickly try out multiple different checkpoints for the **same** task. To do so, it's enough to pick one `Auto...` model class such as `AutoModelForCausalLM` together with, *e.g.* `AutoTokenizer`. I don't see at all the need to be able to quickly switch between different tasks in the same script. If one wants to switch for a `language-generation` task to let's say speech classification, I don't think the convenience of not having to change `AutoTokenizer` to `AutoFeatureExtraction` is worth much compared to the complexity added to this function. 

iiii) The idea here is really that `AutoProcessor` can be used for **all** kinds of preprocessing. This might make the user believe that the same holds true for `AutoModel`. But `AutoModel` is different IMO as it only returns the encoder of the models and can **not** really include all models (e.g. RAG, EncoderDecoder, SpeechEncoderDecoder, ...) 

To conclude, I would prefer to have `AutoProcessor` just return `...Processor` objects and neither feature extractors nor tokenizers.

There is one thing, where I see my logic a bit flawed and where I understand why this class is coded the way it is:

a) The "...Processing" name. I agree that all pre- and post- tokenization, feature extraction, etc... can be summarized by the name "processing".

Very interested in discussing this a bit more! 

<|||||>i know nothing about the details of this but from my superficial understanding of this I agree that "I'm not a fan of making it an umbrella class for both tokenizers, feature extractors and processors"<|||||>(note that thanks to @sgugger automated metadata sharing we will soon be able to display an actually sensible sample code for tokenization/preprocessing/etc on the transformers models in the hub)<|||||>I have absolutely no strong opinion on this, I added this because @LysandreJik told me to :-)<|||||>Following the discussion of https://github.com/huggingface/moon-landing/issues/3632 ,

Want to maybe kick-start a discussion here as this PR / issue has been hanging a bit in the air and I think it was mostly me that was blocking this PR and it might be time to unblock it.

Having revisited my points [here](https://github.com/huggingface/transformers/pull/14465#issuecomment-983810350), I guess my opinion changed a bit with regard to:

i) The no-magic philosophy applies a bit less to `Auto....` class I guess since they can now also directly load from the Hub, cover all kinds of models etc... so would not count this as a strong argument anymore. I do think we'll quickly get quite complex code in `AutoProcessor` to handle all the weird use cases

ii) Still feel strong about this as it clearly breaks a pattern to me and is somewhat unexpected to someone that knows `transformers` well IMO (AutoProcessor returns ...Tokenizer) even though there is a `AutoTokenizer` is not clean IMO.

iii) I do see a clearer use case now! So happy to scratch that

iv) think it's also not that big of a deal and think models and processors can just be treated differently

=> so overall if @LysandreJik @thomwolf @sgugger you're more in favor of merging this PR, happy to be outvoted here :-) Don't feel that strongly about it anymore.<|||||>If we pick a different name for the auto class (not `AutoProcessor`), I think it makes ii) a moot point. Since it seems to be your biggest argument against, would that be enough of a compromise for you?<|||||>Yes, also totally fine for me to go with `AutoProcessor` - I do see the easy of use as a big argument that outweighs 2) -> so also ok for me to use `AutoProcessor` if that's the best name here <|||||>Glad to see we're aligned for a good coverage of processors! I personally like `AutoProcessor` and think it's not necessarily unclear if we have some good documentation.

`AutoProcessor` is also the current UI for some models, so the API won't be changed for these models (which is good).