[ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add Qwen3 K16 attention and fp16 KV cache #21136
• [ExecuTorch][WebGPU] Add HuggingFace rotate-half RoPE #21135
• [ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation #21134
• [ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds #21133
• [ExecuTorch][WebGPU] Add Llama K16 online causal attention #21132
• [ExecuTorch][WebGPU] Replace broad QKV fusion with BK64 kernel #21131
• -> [ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM #21130
• [ExecuTorch][WebGPU] Fuse the SwiGLU elementwise chain #21129
• [ExecuTorch][WebGPU] Coordinate dynamic dispatch routes #21128
• [ExecuTorch][WebGPU] Add certified scoped output suppression #21127

Llama prefill drives the ordinary quantized-linear projections at a small set
of fixed batch-row counts, and the generic Steel schedule leaves throughput on
the table for them. This adds a frozen BK64 Steel quantized-GEMM route for the
exact accepted Llama ordinary-projection shapes at live M128, M508, and M512,
selected only when the capability and dynamic-route guards all pass. M511 and
other prefill sizes stay on the generic Steel schedule and M1 stays on bicol
decode, so the route fails closed outside its accepted shapes. Mirrors Vulkan
xplat/executorch/backends/vulkan/runtime/graph/ops/glsl/q4gsw_linear_gemm__w_4x8.glsl
(the 4-bit grouped-symmetric-weight GEMM), specialized here for the BK64 tile.

Key changes:

• runtime/ops/quantized_linear/q4gsw_steel_bk64.wgsl (+ generated header): the
  BK64-tiled q4gsw prefill GEMM kernel.
• QuantizedLinear.cpp, WebGPUUtils.h: exact-shape predicate, capability guard,
  and dynamic re-entry into and out of the BK64 route.
  @exported-using-ghexport

Differential Revision: D113171739

Differential Revision: D113171739

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21130

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Active SEVs

There are 1 currently active SEVs. If your PR is affected, please view them below:

• Unrelated MacOS MPS test jobs time out (30 min) due to stale cpp_extension lock file poisoning self-hosted runners

❌ 1 Cancelled Job

As of commit 385c00c with merge base 86c3470 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-nxp-neutron / linux-job (gh)
  ##[error]The operation was canceled.

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[SS-JIA]

Review automatically exported from Phabricator review in Meta.