[Don't review] webgpu: Refactor SubgroupMatrixMatMulNBits to vendor-agnostic config …

[qjia7]

…+ add NVIDIA 16x16x16

Refactor subgroup matrix MatMulNBits support from vendor-specific (Apple/Intel) to a vendor-agnostic config-based approach. Any GPU reporting a matching subgroup matrix config from Dawn is now automatically supported.

Key changes:

• Replace vendor-specific config table with SupportedSubgroupMatrixConfig struct containing {componentType, resultComponentType, M, N, K, subgroupMinSize, subgroupMaxSize, needsPrepack}. No architecture or backendType required.
• Remove vendor_ member from SubgroupMatrixMatMulNBitsProgram. Shader selection is now driven by config dimensions (8x8x8, 8x16x16, 16x16x16).
• Remove vendor gate in matmul_nbits.cc call site.
• Rename shader templates: _apple -> _8x8x8, _intel -> _8x16x16.
• Add new 16x16x16 shader template for NVIDIA Blackwell (RTX 5080).
  • 4 subgroups x 32 lanes = 128 threads per workgroup
  • 64x64 tile with 16x16 subgroup matrices
  • Bounds-checked output via scratch buffer for partial M tiles
• Fix prepack shader OOB reads: add scalar fallback with zero-fill for partial blocks where M is not a multiple of kSgMatM.
• Prioritize larger configs (16x16x16 > 8x16x16 > 8x8x8) when multiple match.

Verified on NVIDIA RTX 5080 (Blackwell, Vulkan backend):

• Correctness: model-qa.py with phi4-graph-prune produces identical output to D3D12 baseline
• Prefill (phi4, l=1024):

phi4-graph-prune	D3D12 DP4A	Vulkan DP4A	Vulkan TC (16x16x16)
Prefill (tps)	3,134	6,389	7,089

• NVIDIA reports ChromiumExperimentalSubgroupMatrix with F16/F16 16x16x16 config

Description

Motivation and Context