[Sampler] Support returning all logprobs or logits

[22quinn]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Often for numerical benchmark and debugging purpose, we need to get all the logits or logprobs (controlled by logprobs_mode). This PR supports returning all of them, of vocab_size length, if SampleingParams.logprobs=-1 and ModelConfig.max_logprobs=-1

Test Plan

pytest tests/v1/sample/test_logprobs.py -k 'test_all_logprobs'

Test Result

Test passed

(Optional) Documentation Update

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request introduces support for returning all logprobs or logits by setting logprobs=-1. The changes are well-contained and include updates to configuration, validation, and worker logic, along with a new test case.

My review identified a critical bug in the validation logic. The current implementation would incorrectly allow a request with logprobs=-1 even when the engine has a max_logprobs limit set, which could lead to unexpected resource consumption. I've provided a code suggestion to fix this validation logic. The rest of the implementation appears correct and effectively adds the desired functionality.

[DarkLight1337]

cc @njhill

[houseroad]

why do we want to return all the logits? This can easily run into OOM, right?

[njhill]

This looks reasonable to me but would be good to get some more opinions.

why do we want to return all the logits? This can easily run into OOM, right?

@houseroad yes I think it could cause significant performance problems in a multi-user production scenario but imo it's ok to support this for experimental purposes given that it needs to be explicitly enabled via the config.

[22quinn]

yea this is not for production but nice to provide an option for power users to get everything they want (e.g. for debugging).
Btw test failures look irrelevant

[houseroad]

Sure, I am fine with opt-in solution then. :-)

[houseroad]

Let's add a comment to explicitly call out that -1 is likely causing OOM?

[22quinn]

sounds good, updated