vectorize PrefixConstrainedLogitsProcessor
# What does this PR do?

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Fixes #25217 (in part).


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- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@gante 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25276). All of your documentation changes will be reflected on that endpoint.<|||||>There's a silly shape thing happening here which I'll try to debug ASAP (unless others are interested). Unfortunately testing locally is not working since I'm on Silicon and some dependencies for dev aren't available ☹️  but this looks close. I'll want to think hard about the vectorization of the function (which is slightly different and hopefully not breaking).<|||||>@erip thank you for jumping into the issue 💪  LMK when it is ready for review (assuming it yields speedups)<|||||>I believe it'll yield some improvements since there will be much less CPU<->GPU with masking ops. Whether they're significant will be hard to measure. My big concern is that the semantics of the prefix fn will change slightly (reflected in the test); whether this is acceptable is unclear.<|||||>Worst case scenario, a flag could be set at init time (of the logits processor), if the function supports vectorization<|||||>cc @gante I think this is ready for review. Nothing too controversial here, but I can add a fallback to original behavior in case the fn doesn't support vectorization. I'd like to test the speedup eventually, but I think this won't incur regressions at the very least.