[Spec][PP] Support MTP speculative decoding under pipeline parallelism (PP>1)

[atassis]

Purpose

Enable MTP speculative decoding under pipeline parallelism (PP > 1), greedy-equivalent to the no-spec baseline. This path used to crash or silently diverge: the speculative-decode token accounting is computed on the last PP rank (where the sampler lives) and never reached the non-last ranks, so they ran on stale, optimistic state.

I validated it on current main: greedy-equivalent across MiMo (pure attention) and Qwen3.5-27B (hybrid GDN) at num_speculative_tokens 1-3, including mamba_cache_mode=align and fp8 KV cache, at 1.68-1.89x. Implements the design in #44697; closes #36643, closes #36872.

Root cause

Spec decode advances num_computed_tokens optimistically (assuming all drafts are accepted) and corrects it after the forward via the GPU kernel update_num_computed_tokens_for_batch_change (gpu_model_runner.py:2115), gated on valid_sampled_token_count_gpu (:2107). That tensor is only produced by the sampler, i.e. the last PP rank. On non-last ranks it's None, so the correction is skipped, positions over-advance by the rejected-draft count after every rejection, rope/KV goes off by one, and verification produces non-greedy output.

Two more pieces have the same shape: hybrid (GDN/mamba) models roll back conv1d/SSM state by num_accepted_tokens (set in _update_states_after_model_execute :1529, last-rank only), and the sampled-token / draft values the non-last ranks need to embed the next input are also only resident on the last rank. One invariant fixes all three: the non-last rank applies the sampler's broadcast per-request valid/accepted count to its own accounting.

Why this isn't a duplicate

• [Bugfix][Scheduler] Fix CUDA crash caused by stale async placeholder tokens in speculative decoding #40768 fixes the scheduler placeholder crash; it doesn't fix non-last-rank correctness or PP greedy-equivalence. It's complementary, not a dependency: I checked by reverting it and greedy-equivalence still holds, including at batch=16. I don't re-implement it here.
• Feat:add support for PP and MTP #39704 / [bugfix] DeepSeek MTP support and Pipeline Parallel feature. #38104 are earlier PP+MTP attempts, both untested and currently conflicting. This adds verified greedy-equivalence, hybrid (GDN/mamba) support, the draft-memory enablement, and a full test + benchmark suite.

Changes (6 commits)

1. Experimental gate: a one-time logger.warning_once when MTP runs with PP > 1, so users know the path is new while V2 catches up.
2. Draft memory enablement: int4 draft-embed load-time quant + skipping the draft lm_head, so the draft fits on the last PP rank for large models. Draft-only, so it can't change output; int4 measured == int8.
3. Width-agnostic broadcast (vllm/v1/worker/pp_spec_broadcast.py): typed transport of the sampler's per-request tokens/counts to the non-last ranks (gloo-tested).
4. Non-last-rank input reconstruction: write the real broadcast values into token_ids_cpu (never leaving -1), scatter draft tokens into spec positions, pad the sender width. Fixes the crash.
5. num_computed_tokens drift correction: reconstruct the skipped correction on the non-last rank from the broadcast valid count, in _update_states.
6. num_accepted_tokens for GDN/mamba: source the accepted count from the broadcast so non-last GDN layers roll back state identically. Gated on is_hybrid.

Test Plan

# Unit (CPU, no GPU):
.venv/bin/python -m pytest \
  tests/v1/spec_decode/test_pp_spec_broadcast.py \
  tests/v1/spec_decode/test_quantized_draft_embedding.py \
  tests/v1/spec_decode/test_pp_draft_config.py \
  tests/v1/spec_decode/test_qwen3_5_mtp_standalone.py \
  tests/v1/spec_decode/test_draft_embed_quant_integration.py -q
pre-commit run --all-files

# Integration (2 GPU): spec output token-identical to the same-config no-spec baseline,
# on MiMo (pure attention) and Qwen3.5-27B-AWQ (hybrid GDN, cpu_offload_gb=3, int4 draft).

Test Result

• Unit: 31 passed (CPU; includes the 2-rank gloo broadcast round-trip and int4 draft-embed quant). ruff check and ruff format pass on all changed files.
• Integration (current main, 2x 16 GB GPUs): greedy-equivalent on every config I tried, 1.68-1.89x, ~94% acceptance on 27B.

Full validation matrix

• MiMo PP=2 + MTP: 5/5 token-identical at k = 1, 2, 3 (so the fix generalizes past k=1); 1.68x at k=1 (38.16 vs 22.75 tok/s).
• Qwen3.5-27B-AWQ PP=2 + MTP (hybrid GDN): 5/5 at k = 1, 2, 3; 1.89x at k=1 (offload-bound absolute tok/s, the ratio is the signal). Also 5/5 with mamba_cache_mode=align and 5/5 with fp8 KV cache (the production-gateway regime).
• Sampling: temperature 0.8 with a fixed seed is deterministic (two runs identical) and exact length, so there's no race under stochastic rejection sampling.
• Chunked prefill: greedy-equivalent (long prompt, max_num_batched_tokens=64).
• Acceptance: 27B 94.7% per-position (mean accept length 1.95 / 2.58 / 3.22 at k=1/2/3); MiMo 82.5% at k=1, per-position falls off fast (k=3: 0.81 / 0.14 / 0.01), so length saturates around 2 and k=2 is the sweet spot.
• 256-token runs (3-way): PP=2 has 3/5 sequences identical to 256, 2/5 diverge late (@109, @176). A single-GPU MTP run of the same prompts also diverges late, on a comparable set (seq0@25, seq4@161), so PP isn't systematically worse (it's perfect on the sequence where single-GPU diverges earliest), and both share the same near-tie (seq1@176). The residual is the floating-point near-tie floor of MTP spec decode, not something PP-specific.
• Concurrency (batch=16, no scheduler change): no crash, and exactly the requested length on all 20 sequences. Near-tie divergences are comparable to single-GPU (PP 7/20 seqs vs single-GPU 8/20). This is the regime where [Bugfix][Scheduler] Fix CUDA crash caused by stale async placeholder tokens in speculative decoding #40768's placeholder discipline would matter if it were load-bearing here, and it isn't, which is why this complements [Bugfix][Scheduler] Fix CUDA crash caused by stale async placeholder tokens in speculative decoding #40768 without depending on it.

Relationship to MRV2 (V2 model runner)

V2 is the longer-term home for spec-under-PP (#42538, #43732). I checked it directly: with VLLM_USE_V2_MODEL_RUNNER=1, MiMo PP=2 + MTP loads both ranks and sizes the KV cache but then deadlocks during engine construction (EngineCore shm_broadcast stuck, Worker_PP1 in futex_wait, never reaches generate). So V2 doesn't cover this config yet, and #36643 / #36872 have no working path today without this V1 fix. The invariant here is small and should port cleanly to V2's PPHandler, and I'm happy to align with whatever V2 prefers.

Notes

• AI assistance was used in diagnosing, implementing, and drafting this PR. I reviewed every changed line and ran the tests above myself.
• Out of scope: mamba_cache_mode=all (none and align are verified); draft PP > 1 (the draft stays on one stage); EOS-triggered early stop (the validation base model emits no EOS, so only the ignore_eos exact-length path is exercised).

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @atassis.

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[atassis]

Hi @njhill — you're a codeowner here and the MRV2 author, so flagging this one: a V1 fix for MTP + PP>1, greedy-equivalent and validated on MiMo and Qwen3.5-27B. Could you take a look when you have a moment, or point me to the right person? Happy to align with whatever V2 wants.