[Bugfix] Padded Eagle Specdec with Chunked Prefill

[Flechman]

Purpose

This adds test to #26231

Test Plan

Test Result

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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.
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[gemini-code-assist]

Code Review

This pull request provides a bugfix for padded speculative decoding, specifically within the prepare_next_token_ids_padded function. The change removes a faulty special case for single-token generation (max_gen_len == 1) that did not correctly handle discarded requests. This could have resulted in invalid token IDs being used in subsequent steps. The new implementation generalizes the validity masking logic, making it simpler, more robust, and correct for all scenarios. The fix is sound and improves the correctness of the speculative decoding implementation.

[gemini-code-assist]

This simplification is a great improvement. By removing the special case for max_gen_len == 1, the code is now more robust. The previous logic didn't account for discarded requests when max_gen_len == 1, which could lead to using an invalid token ID of -1. This unified approach correctly handles all cases.

[benchislett]

This bug has been fixed in #26231. I think it would still be nice to merge updated tests, so please update your PR with the fix from main if you wish to continue.

[Flechman]

@benchislett sounds good! Done.

[benchislett]

LGTM

[Flechman]

@benchislett I enforced the test to run on GPUs. It passes locally with 4xH100.