Expose blip2qformer

[alex-jw-brooks]

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).

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).
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• Did you make sure to update the documentation with your changes? Here are the
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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.

@ArthurZucker can you please take a look?

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