[Bugfix] Handle best_of>1 by disabling multi-step scheduling; fail if beam search is invoked with multi-step scheduling

[afeldman-nm]

This PR addresses #7968 by disabling multi-step scheduling when best_of>1 is enabled; this is the same approach as was taken in #6138 which disabled speculation for best_of>1. This prevents requests with best_of>1 from failing with an unhandled exception, as is the current case.

Since beam search has been moved outside the engine, this PR causes beam search to fail when it is invoked with multi-step.

FIX #7968 (link existing issues this PR will resolve)

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[comaniac]

I understand the approach but get a bit confused about the code changes. Specifically how the "disabling multi-step when n>1" handled in this PR? Seems like this PR only raises an exception?

[afeldman-nm]

Apologies @comaniac forgot to mark this as draft

[comaniac]

Approved to unblock this PR first. Per offline discussion we still need to change the output processor interface for this feature, but maybe we could have a better argument name.

[njhill]

@afeldman-nm is this still required given the work to remove the impl of best_of and beam search from the core engine? Another advantage of moving the impl of these outside of the engine is that they should still work with multi-step (though the performance might not be great)

[afeldman-nm]

Hi @njhill can you please point to the PR or other information about the workstream which would move best_of outside of the engine?

[njhill]

@afeldman-nm this is the one I was thinking of, from @youkaichao: #9302

and #9261

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @afeldman-nm.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork