[`bnb`] Fix blip2 4bit
# What does this PR do?


Fixes https://github.com/huggingface/transformers/issues/23839

Indeed, for models such as Blip2 that have lm head inside submodules (and not directly on the top level of the model), the lm head does get converted to 4bit / 8bit models, leading to unexpected behavior for 4bit models. The PR fixes this by making sure to consider the last term after `.` when creating `modules_not_to_convert`.

cc @sgugger 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23895). All of your documentation changes will be reflected on that endpoint.<|||||>It should be all good, I have verified the slow tests pass for 8bit and 4bit. Let me know if there is anything particular I should have a look. Per my understanding this only affects Blip2 as it is the only model (from what I know) that have an lm head as part of a submodule.<|||||>Hmm getting some gibberish output with the fix, need to investigate more<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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