[RFC][Spec Decode] Drive structured-output masks from GPU logit counts

[LucasWilkinson]

Purpose

Alternative implementation for vllm-project#52436. Instead of teaching the CPU mapping about the adaptive zero-budget special case, make structured-output mask selection depend on the GPU's finalized per-request logit counts.

The scheduler now emits a fixed-width mask block for every structured-output request. The Triton kernel launches per grammar request, resolves the request's active logit range from the GPU-side cu_num_logits, and applies only the active positions from that request's mask block.

This removes the dependency on CPU cu_num_logits_np, draft counts, and bonus counts from the mapping path. It also avoids adding layout metadata to InputBatch or GrammarOutput.

The legacy V1 runner is updated to read the same fixed-width blocks while retaining its existing packed-logit behavior.

Relationship to the parent PR

This is deliberately stacked on vllm-project#52436 and replaces its host-side scheduled-draft mapping with a GPU-driven design; it is not a second independent fix for the same upstream bug.

Duplicate-work searches for 52436 in:body and adaptive verification structured output grammar bitmask found no other PR addressing this design. The parent PR has no linked issue.

Test plan

.venv/bin/python -m pytest \
  tests/v1/spec_decode/test_adaptive_verification.py \
  tests/v1/spec_decode/test_mtp_structured_output.py -q
# 22 passed

.venv/bin/python -m pytest \
  tests/v1/structured_output/test_backend_guidance.py -q
# 6 passed

.venv/bin/python -m pytest \
  tests/v1/worker/test_gpu_rejection_sampler_chunking.py \
  tests/v1/worker/test_gpu_batch_ordering.py -q
# 15 passed

.venv/bin/pre-commit run --files \
  tests/v1/spec_decode/test_adaptive_verification.py \
  tests/v1/spec_decode/test_mtp_structured_output.py \
  vllm/v1/structured_output/__init__.py \
  vllm/v1/structured_output/utils.py \
  vllm/v1/worker/gpu/model_runner.py \
  vllm/v1/worker/gpu/structured_outputs.py \
  vllm/v1/worker/gpu/warmup.py
# all hooks passed, including Ruff and mypy

A direct Triton smoke test covered two grammar requests with a fixed stride of four and GPU counts [1, 1], verifying that only mask rows 0 and 4 were applied and inactive rows were ignored.

Model evaluation

Ran a full DeepSeek-V4-Flash-0731 TP=4 generation with DSpark adaptive verification forced to a zero draft budget and two simultaneous JSON-schema requests:

CUDA_VISIBLE_DEVICES=0,1,2,4 .venv/bin/python \
  .venv/repro_strided_grammar.py

The local repro used two dense eight-row mask blocks, observed one active GPU logit per request, and completed with valid structured outputs:

{"answer": 2}
{"answer": 3}

AI assistance

AI assistance was used to analyze the bug, implement this alternative, and run the tests above. This remains a draft pending human review of every changed line before it is marked ready.

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