[Bugfix][v1] Fix step pooler implementation and step pooling usage in v1

[Isotr0py]

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)".
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Purpose

• Fix redundant pooled_data = [] in step pooler implementation
• Fix missing prompt_token_ids in pooling metadata for step pooling when using v1 runner

Test Plan

pytest -s -v tests/models/language/pooling/test_reward.py

Test Result

• Test should pass on both v0 and v1 engine

(Optional) Documentation Update

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[gemini-code-assist]

Summary of Changes

Hello @Isotr0py, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request focuses on critical bug fixes related to the step pooler implementation and its usage within the v1 runner. It resolves an issue where collected pooled data was being inadvertently discarded and ensures that necessary prompt token IDs are correctly propagated for step pooling operations, enhancing the correctness and reliability of the pooling mechanism.

Highlights

• Step Pooler Implementation Fix: Corrected a logical error in vllm/model_executor/layers/pooler.py where the pooled_data list was redundantly re-initialized after being populated, ensuring that collected pooled data is no longer discarded.
• V1 Runner Step Pooling Metadata: Addressed a bug in vllm/v1/worker/gpu_input_batch.py to ensure that prompt_token_ids are correctly included in the sampling metadata when step pooling is active, which was previously missing.
• Conditional Prompt Token Handling: Introduced a new is_step_pooler flag in vllm/v1/worker/gpu_model_runner.py and propagated it to the InputBatch constructor, allowing for conditional inclusion of prompt_token_ids specifically for step pooling operations.

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[gemini-code-assist]

Code Review

This pull request fixes a bug in the step pooler implementation and ensures that prompt token IDs are correctly handled when using step pooling with the v1 runner. The changes include removing a redundant assignment in the pooler, adding a flag to indicate the need for token IDs during sampling, and passing this flag to the InputBatch constructor and reinitialization methods. These changes improve the correctness and efficiency of step pooling.

[Isotr0py]

Also cc @maxdebayser

[maxdebayser]

Thanks for the fix! Overall it looks good to me. I've only requested a few small refactorings.

[maxdebayser]

LGTM, thanks!