[Model][PP] Add DeepSeek-V4.1 sequence-parallel stage boundaries

[0z5a]

Purpose

Add the default-off --additional-config '{"deepseek_v41_pp_sp":true}' path. Gather and trim both hidden states and delayed pre_mix at a pipeline exit, then shard both at the next stage. Apply Engram at a stage's first decoder layer before its attention pre-mix. Initialize single-DP sequence-parallel MoE communication before groups are created; require EP, TP≥2 and, for DP=1, allgather_reducescatter.

Includes the small local cache-allocation prerequisite from #56221. Cross-stage sharing is separate in #56223; the upstream Engram all-gather implementation is retained. The duplicate check found #53191 for DeepSeek-V2/Qwen3 models; it does not handle the V4.1 delayed mHC pre_mix and Engram stage boundary addressed here.

Based on #56214 (dsv41-feat, c9d909e802a39292f54101bff8a36096761ea605). The PR targets that feature branch so the model implementation is not repeated in this diff.

Rebased on 2026-09-11 after the feature branch was rewritten. Changed-file Python parsing and all applicable pre-commit hooks passed. GPU/full-model measurements below belong to the prior revision based on e47aa780bccf59f59dfa2cbb18e17a10b4fe69ba; they were not rerun on this rebased head.

AI assistance: OpenAI Codex assisted with implementation, review, test execution and preparation of this PR. This draft does not claim that a human has completed a line-by-line review.

Test Plan

Fresh split: all applicable pre-commit hooks passed for the six changed files; all changed Python files parsed successfully. The new cases in tests/test_config.py cover disabled/enabled DP=1 communication and rejection of an unvalidated collective backend. Those three cases passed in the earlier H100 integration, together with the cache-group checks; they were not rerun as GPU/model tests on the split branch.

Test Result

Recorded full-model runs used deepseek-ai/DeepSeek-V4.1-Flash, revision df42c109f1defefcbfcedbe7d905718a12266e40, on 4×SXM H100 80 GiB. Each positive matrix case checked six short questions, cold/cached long input with chunked prefill, and four unequal-length concurrent requests. “Text equal” means short-answer text equality to the TP4 reference, not logits/token-probability parity or a standard accuracy benchmark.

Evidence scope: GPU results below come from the earlier combined integration based on #56214, including companion changes. The isolated PR has not had a new full-model GPU run. Fresh split checks are listed separately; passing the integrated run does not establish isolated-branch equivalence.

Integrated configuration	QA answers correct	QA text equal to TP4	Long / chunked + prefix	Concurrent requests	Strict result
PP2×TP2 SP eager, 20/20, synchronous lookup	6/6	5/6	2/2 correct; cache reuse checked	4/4 correct	Text mismatch; strict check not passed
PP2×TP2 SP eager, 20/20, lookup overlap (#56220)	6/6	6/6	2/2 correct; cache reuse checked	4/4 correct	PASS
PP2×TP2 SP + sharing (#56223), 19/21, synchronous	6/6	6/6	2/2 correct; cache reuse checked	4/4 correct	PASS
PP2×TP2 SP + sharing + overlap, 19/21	6/6	5/6	2/2 correct; cache reuse checked	4/4 correct	Text mismatch; strict check not passed
PP2×TP2 SP breakable decode graph, synchronous	6/6	5/6	2/2 correct; cache reuse checked	4/4 correct	Text mismatch; strict check not passed
PP2×TP2 SP breakable decode graph, overlap option	6/6	5/6	2/2 correct; cache reuse checked	4/4 correct	Text mismatch; strict check not passed

All four workers confirmed that SP was active. Graph rows completed capture and generation with FULL_DECODE_ONLY, sizes [1,2,4], and Torch compile disabled. Only 2/6 configurations passed the strict short-answer text comparison. The remaining four differed on the sequence question's wording; the cause is unresolved. This remains Draft for isolated-branch GPU/output validation.

Short-request serving with the companion sharing implementation

Integrated configuration	Input → output	Successful requests	Concurrency	Output tokens/s	Median TTFT / TPOT ms
PP2×TP2+SP + #56223 sharing, 19/21 layers	512 → 128	16/16	4	11.458	8235.85 / 294.05

One measured run after warmup, eager, prefix cache off, CPU weight offload 8 GiB. This is an absolute result for a configuration that requires sharing; disabling sharing rejects this partition. It does not establish an A/B speedup attributable to SP. No measured GPU-memory reduction is established for PP+SP.

Validation	Result	Origin
SP configuration / collective initialization	3/3 PASS	Earlier H100 integration
Projected-cache-group regression	2/2 PASS	Earlier H100 integration; prerequisite from #56221
Applicable pre-commit hooks and changed Python parsing	PASS; 6 changed files	Fresh isolated split

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PR description checklist

• Purpose and related work described.
• Test plan and actual results stated.
• Model evaluation limitations stated.
• AI assistance disclosed.

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[mergify]

Documentation preview: https://vllm--56222.org.readthedocs.build/en/56222/

[claude]

Claude Code Review

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[0z5a]

Validation follow-up for 3dc4d14b5e3bd564d73460635c6e1d3d9d231d27: the latest H200 attempt exercised SP-off and SP-on functional smoke, but did not preserve a complete, independently auditable A/B result pair. The older combined configurations also contain unresolved output differences. Those observations do not establish isolated current-head equivalence or an SP speedup.

A useful next run is PP2×TP2 with the same legal 20/20 partition, SP off/on, sharing and lookup-overlap held fixed, and identical checkpoint/offload/cache settings. Please verify the hidden-state and delayed pre_mix stage boundaries, first-layer Engram handling, padding, long/chunked prefill and prefix reuse, with all workers attesting the intended SP path. Add repeated serving measurements and actual per-rank memory records if making performance or memory claims. The 19/21 sharing-dependent configuration is not an SP-only A/B control.

Contributions from anyone with access to sufficient GPU capacity are very welcome: reproducible GPU E2E benchmarks, targeted correctness checks, and focused fixes. Please include source/model revisions, GPU topology, offload/KV settings, exact commands, and raw results; repeat timing runs when making performance claims. I am happy to review and cherry-pick fixes while preserving the original authorship and clear attribution.

[0z5a]

Resubmitted as #56438 against main, since dsv41-feat was merged into main in #56214.