fix(dspark): reserve V2 padded Q scratch

[alexbi29]

Summary

• Fixes the DSpark V2 runner OOM by removing the runtime padded-Q allocation from fused_deepseek_v4_qnorm_rope_kv_rope_quant_insert.
• The fused op now writes into caller-owned q_out; DeepSeek V4 attention pre-reserves reusable padded-Q scratch during V2 profile/runtime.
• Updates functionalization metadata and fused-kernel tests for the new mutating op signature.

Validation

Check	Result
Live V2 startup	Using V2 Model Runner, GPU KV cache 1,095,149 tokens, max concurrency 2.09x
Long-context concurrency	No OOM / no HTTP 500; concurrency-8 at ~29.6k prompt tokens, 15/16 exact recall, one retrieval miss
TPS sanity	avg 303.8 tok/s, median 302.0, range 297.4-311.5

[alexbi29]

@jasl this should fix v2

[jasl]

Thanks @alexbi29 — validated end-to-end on 2-node GB10 (TP=2, DSv4-Flash, FULL_AND_PIECEWISE) and merged. The fix is correct and does exactly what it says. Because the changed callsite (_fused_qnorm_rope_kv_insert) is shared with the V1 default runner + the MTP draft path, I ran the full V1/MTP matrix too, not just V2:

V1 (production default)

• GSM8K 0.975 · issue19 instruction-following PASS · smoke coherent
• Arthur needle-recall conc12 @~28k (batched FULL cudagraph): 24/24; @~434k conc1: 2/2
• Memory: GPU KV cache 2,741,470 tok = pre-PR baseline exactly, no post-profile OOM (the persistent scratch does not shrink KV)

V1 + MTP2

• GSM8K 0.953 · arthur conc12 24/24 · acceptance rate 0.91 / accept-len 2.57 (healthy)
• decode 29.5 tok/s c1 / 48.8 c4, TPOT ~25–32 ms — perf-neutral (the change removes a per-forward alloc)

V2 runner — and this is the interesting part:

• No OOM at conc8 / ~30k (the fix's purpose) ✓
• Arthur recall conc8 @~28k: 16/16, GSM8K 0.96

That 16/16 is worth flagging. We had previously recorded V2 long-context recall collapsing at this exact config (we measured 3/16 @28k/conc8) and had written it off as an unfixable cudagraph-replay-class recall bug — the reason we'd stayed on V1. Your padded-Q OOM fix also resolves that collapse: the old "misses" were requests erroring under memory pressure, not a replay bug. So this reconciles your 15/16 vs our 3/16 — with the fix it's a clean 16/16 here, and V2 is back on the table for us. Nice catch.

Two small follow-ups I applied/noted on the merge (not blockers):

1. Wired reserve_profile_scratch into the V1 runner too (it was V2-only). On V1 with enable_dbo=true, the ubatch-1 padded-Q scratch would otherwise be materialized after KV sizing (during ubatched capture) and charged against already-claimed KV → post-profile OOM risk. No-op for the DBO-off default (num_ubatches=1). Added as a follow-up commit so the shared branch is correct for all configs.
2. tests/compile/passes/test_functionalization.py still registers the stale 3-arg torch.ops.vllm op, so CI only exercises the {1:q,2:k_cache} else-branch — the production _C {1:q_out,2:k_cache} branch (which the mutable-arg order in torch_bindings.cpp makes provably correct) is never tested. Worth updating the test op to the 4-tensor void _C schema to lock it against future schema drift.

Thanks again — merged into codex/ds4-sm120-min-enable (= this PR's base) + ds4-sm120-preview-dev.