Expose blip2qformer
Author: alex-jw-brooks
Pull request: yes

This PR exposes Blip2QFormer so that it can be created via `AutoModel.from_config`, which is needed for making the projector component in granite speech generic (discussed in [this PR](https://github.com/huggingface/transformers/pull/36801)).

Blip2QFormer is most of the way there already - this PR just:
- Handles AutoModel registration
- Adds docstrings / more information on forward inputs

Note that Blip2QFormer is already tested pretty extensively though the blip2 tests, and will also be tested through granite speech tests.

## 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#create-a-pull-request),
      Pull Request section?
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      to it if that's the case.
- [ ] 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).
- [ ] 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:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
- speech models: @eustlb
- graph models: @clefourrier

Library:

- flax: @gante and @Rocketknight1
- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- pipelines: @Rocketknight1
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- tokenizers: @ArthurZucker
- trainer: @zach-huggingface and @SunMarc
- chat templates: @Rocketknight1

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- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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@ArthurZucker can you please take a look?

Comments:
Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. The CI will be paused while the PR is in draft mode. When it is ready for review, please click the `Ready for review` button (at the bottom of the PR page). This will assign reviewers and trigger CI.

Awesome, thank you for the quick review @zucchini-nlp! I went ahead and rebased this pr to fix the conflict 🙂 

Thanks, we can merge when CI turns green