Refactor image features selection in LlaVa
Author: kenza-bouzid
Pull request: yes

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Wrap image features selection in LLaVa in a separate function to make it easier to override for custom use cases (e.g. applying a layer norm on the image features before projection, etc).

Fixes #33695 

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Comments:
Hi @zucchini-nlp, 

Thanks for your reply. 

>WDYT about moving the whole image feature preparation to a separate method, including MM-proj. So the method takes in pixels and return features ready to be merged with text embeddings?

Good idea, however I would suggest having a separate method for that as well to make it easy to customize the projection. I can think of many use cases where you may want to change the projection.
  
We can have `get_image_features` and `project_image_features`  that are then called sequentially in a `get_image_embeddings` ready to be merged with text embeddings as you suggested. 

> If you have bandwidth, would be nice to propagate change to all VLMs. But don't stress if you can't, I'll make an overall standardization soon on that :)

I'm afraid I won't have time to propagate the change to all VLLMs. Is that a blocker for this PR to pass the CI? You probably have more context for an overall standardization! 

> I would suggest having a separate method for that as well to make it easy to customize the projection. I can think of many use cases where you may want to change the projection.

Hmm, imo that is a bit redundant, and since "select+proj" is mostly interlinked and isn't a huge piece of code we may ask users to override that one method, Independently of whether they tweak at selection stage or projection stage. So that we don't end up with separate methods that perform a one-liner, which imo is a bad practice. Lmk if you have any other ideas or objections

> I'm afraid I won't have time to propagate the change to all VLLMs. Is that a blocker for this PR to pass the CI? You probably have more context for an overall standardization!

No worries, I just realized it is the very first llava being modified here. So our CI should be happy after running `make fix-copies` and `make style`. For other VLMs that are more tricky, I'll make one round of standardization later. Not a blocker at all  :)

Sounds great! Let me make the changes you suggested. Thank youu!

@zucchini-nlp can you please approve/trigger the CI workflows for tests? Thank you!

@LysandreJik can you please have a look? Thanks 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33696). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.