[Bugfix] Unify rank computation across regular decoding and speculative decoding

[jmkuebler]

Currently without speculative decoding the rank is computed as with an > (

vllm/vllm/model_executor/layers/sampler.py

Line 801 in baaedfd

result = (x > vals[:, None])

)
whereas with SD it seems it is done via an >= and adding a +1 (

vllm/vllm/spec_decode/util.py

Line 46 in baaedfd

sampled_token_ids_ranks = (logprob_tensor >=

)

Unless I am missing something it seems reasonable to use the same conventions across the package. This PR implements the same logic. The test I added fails on current mainline, and passes with the proposed change. So please first check whether you think that this test should pass. If not, the PR is obsolete.

Also discussed in #7627

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[jmkuebler]

@cadedaniel Here is a small PR to unify the rank computation as we discussed here:

#7627 (comment)

Since this change might slightly change the user facing behavior, do we need to take some special care?

[cadedaniel]

Looks good, thanks! see comments

[cadedaniel]

add a comment on why we add?

[jmkuebler]

In the regular decoding it was just like that as well. Since you already approved I did not change it so you don't need to reapprove.
But happy to add a comment of course if needed.

[cadedaniel]

is there a better way we can do this besides importing a private function? maybe copying one of these https://github.com/vllm-project/vllm/blob/main/tests/spec_decode/e2e/test_logprobs.py ?

[cadedaniel]

actually this will be faster to test, let's keep it as is

[jmkuebler]

/ready

[jmkuebler]

@cadedaniel anything left to do from my side? Or just wait for the merge?

[cadedaniel]

Merged. Thanks!