[Bugfix] DiffusionGemma: only pop a request's logprobs when it commits (#45689)

[waynehacking8]

Purpose

Fixes #45689.

In a mixed decode batch, DiffusionSampler.__call__ gated logprobs
reassembly on the batch-wide is_committing.any(), then popped
self._pending_logprobs[slot] for every decode request that had a stashed
entry — without checking whether that request was itself committing this
step:

if is_committing.any() and self._pending_logprobs:
    for i in range(num_reqs):
        ...
        if is_decode_np[i] and slot in self._pending_logprobs:   # no per-request committing check
            lp = self._pending_logprobs.pop(slot)

So when request A commits while a co-batched request B merely converged this
step (its logprobs stashed via just_converged, but is_committing[B] is
False), B's stash is consumed one step early. B's eventual committed response
then returns fewer logprob rows than tokens, crashing the OpenAI chat formatter
with IndexError: list index out of range (in _create_chat_logprobs).

Credit to @masterFoad for the root-cause analysis and the suggested fix in the
issue.

Fix

Pop a request's stash only when that request commits: build the set of
committing slots from the pre-step is_committing snapshot and require
slot in committing_slots. A converged-but-not-committing request keeps its
stash until its own commit. The commit-loop assembly is extracted into a
_assemble_committed_logprobs static helper so the interleaving is unit-testable
without a GPU. No behavior change for the single-request / aligned cases.

Not a duplicate

• gh issue view 45689 --comments → unassigned, no prior PR.
• gh pr list --state open --search "45689" → none.
• The reporter's other open PRs are all in spec-decode (a different subsystem); no competing PR for this fix.

Test plan / results

New tests/models/test_diffusion_gemma_logprobs.py (pure CPU, no GPU):

$ python -m pytest tests/models/test_diffusion_gemma_logprobs.py -q
4 passed

Covers: only-committing-pops, both-committing-emit-in-order (incl.
cu_num_generated_tokens), no-committing-returns-None, non-decode-skipped.
Verified the test catches the bug: removing the slot in committing_slots
guard makes test_only_committing_request_pops_its_logprobs and
test_no_committing_request_returns_none fail (the early/erroneous pop).

pre-commit run --files vllm/model_executor/models/diffusion_gemma.py tests/models/test_diffusion_gemma_logprobs.py → all hooks pass (ruff check + format, mypy-3.10, SPDX, …).

Notes

AI-assisted: this change was developed with AI assistance (Claude) and reviewed
end-to-end by the submitter.