vulkan: add dedicated PQ2_0 mat-vec and BC-250 batch tuning

[renovys]

Overview

PQ2_0 still uses the generic float mat-vec shader. On an AMD BC-250 (gfx1013/RDNA1, RADV, no cooperative matrices or accelerated integer dot), this dominates decode time. This adds a dedicated PQ2_0 shader and tunes the BC-250 path. It complements the PTQ1_0 work in #252 and extends the BC-250 tuning discussed in #229.

The shader reads each 34-byte PQ2_0 block through aligned 16-bit words, decodes each row once before looping over columns, and computes d * (sum(q*y) - sum(y)). This algebraic reassociation changes floating-point rounding relative to the generic shader; tolerance-based operator tests pass on BC-250. The shader arithmetic matches the earlier deployed implementation used in our reference-comparison tests.

BC-250 uses a 32-thread workgroup/subgroup and eight rows per workgroup (16 for five columns). For PCI 1002:13FE / RDNA1 / no coopmat only, PQ2_0, BF16 and Q8_0 can use mat-vec through 16 columns. BF16/Q8_0 cover the supporting MTP operations; their existing shaders are reused. Other devices keep the existing launch geometry and eight-column limit, but PQ2_0's float mat-vec shader itself changes on them too.

The PR excludes our packed FP16 prefill path, GDN/state-copy fusion, runtime tuning knobs, and measurement scripts.

Additional information

Based on prism at adfffbe41; patch commit 031212ec9. Both were built directly on BC-250 with GCC 16 and Vulkan enabled, Release, identical flags.

llama-bench (mean of 3)	upstream	this PR
tg128, tokens/s	22.638 +/- 0.042	28.324 +/- 0.022 (+25.1%)
pp512, tokens/s	141.714 +/- 0.029	141.695 +/- 0.016

Model: Ternary-Bonsai-2-27B-PQ2_0-MTP-Q8_0; MTP is not used by this llama-bench run. -ngl 99 -fa 1 -ctk q8_0 -ctv q4_1 -b 2048 -ub 512 -t 6 -p 512 -n 128 -r 3.

test-backend-ops test -b Vulkan0 -o MUL_MAT,MUL_MAT_ID -p 'type_a=(pq2_0|bf16|q8_0)' passed on both builds. Upstream: 476/476 tests passed; this PR: 514/514 tests passed. The new coverage includes columns 9 through 16, row tails, batch shapes and the 17408x5120 model shape. These are targeted operator tests. Full CI and an independent model-quality evaluation were not run on this extracted PR.

Only BC-250 hardware is available for testing. The new shader also serves PQ2_0 float mat-vec on other GPUs, including the generated MUL_MAT_ID variants, so independent testing is welcome. No claim of cross-device performance parity is made.

Our earlier deployment tests compared against the previous deployed build: Korean and English/code KL top-1 agreement was 100%, with small perplexity differences. This demonstrates agreement with that reference, not an independent assessment of model quality. English long-context quality and semantic correctness of the real-request sample were not evaluated.

Requirements

• I have read the contributing guidelines.
• AI usage disclosure: YES. Devin SWE-2 max and Codex assisted with implementation, porting, review, benchmarking and this description; Muse max and GLM-5.3 max provided independent review. An earlier operational version also received GLM Flash assistance. The submitted patch and BC-250 test results are recorded separately from the broader deployed optimization stack.