[Pipelines] Extend pipelines to handle multiple possible AutoModel classes
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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This PR extends `pipeline` to better handle multiple auto model classes per pipeline.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
Re:
> Also, IIRC, config.architectures was not added until some point in transformers. Do we have any way to check that we're not breaking legacy models ? (Scanning the hub is my best guess)

Agree that we should be careful here. I'm scanning the hub now to check, but I'm pretty sure that 99% of models have `config.architectures`. Even the very old models like `gpt2` have `config.architectures = [...]` saved. Also, instead of raising an error if `config.architectures` doesn't exist, we could just pick the first element of the tuple -> this way we ensure that we cannot break anything that worked previously. What do you think?

Re:

> However, I have a feeling we're adding another extra layer of complexity.
> 
> Couldn't we use this PR, to simplify the overall logic here. Maybe None could become an empty tuple.
> Single class could become 1-tuple.
> 
> Overall the rest of the flow should be more streamlined, don't you think ?

Agree that we are adding more complexity, but I don't really see how to allow multiple auto classes without adding more complexity. I don't really see how forcing everything to be in the `tuple` format will help here. But keen to hear your proposition on how this could reduce overall complexity!