PP relay correctness fix adopted - prefill triples

Adopted Schaka's relay fix (topk_indices on the hop + index_k_cache guard): TP1xPP6 prefill 5.6k @32k / 7.2k @128k. TP4xPP2 remains decode-optimal (512k c64 129.6, correct 12/12). TP2xPP4 / TP1xPP8 still boot but needle 0/3 (relay cannot handle TP splits) - rejected.

When (PT)2026-09-11 08:09 PT

ServingTP4xPP2, DSpark k=5, fp8_ds_mla, 1M ctx, CUDA graphs

Enginelocalhost/vllm-backport-v41:sm80 (vLLM 0.12.0-sm80 + PR#56201 + Ampere shims + PP relay)

Benchmarks

Metric	Value	Δ vs previous	Unit	Context	Note
`prefill_tok_s` (prefill_tok_s)	5600 tok/s	-2.4%	tok/s	TP1xPP6 @32k (7.2k @128k) - ~2.5-3x TP4xPP2

What didn't work

TP2xPP4 and TP1xPP8: needle 0/3 after relay fix - arbitrary kv-group TP splits still unsupported

raw JSON: /api/reports/17