[ExecuTorch][WebGPU] Test coverage for the packed-word-dequant f16 steel GEMM

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Test coverage for the f16-accumulate steel GEMM #20801
• [ExecuTorch][WebGPU] f16-accumulate (pwdqf16acc) steel q4gsw prefill GEMM #20800
• -> [ExecuTorch][WebGPU] Test coverage for the packed-word-dequant f16 steel GEMM #20799
• [ExecuTorch][WebGPU] Packed-word-dequant f16 steel q4gsw prefill GEMM #20798
• [ExecuTorch][WebGPU] Reuse pool for single-op scratch (-86.7 MiB) #20797
• [ExecuTorch][WebGPU] Test coverage for the f16 KV cache #20773
• [ExecuTorch][WebGPU] Add opt-in native f16 KV cache (−280 MiB, default-OFF) #20772
• [ExecuTorch][WebGPU] Test coverage for the f16-multiply steel q4gsw GEMM #20753
• [ExecuTorch][WebGPU] Add f16-multiply variant of the steel q4gsw prefill GEMM #20752
• [ExecuTorch][WebGPU] Steel q4gsw prefill GEMM — test coverage #20731
• [ExecuTorch][WebGPU] Add 256-thread "steel" q4gsw prefill GEMM #20730
• [ExecuTorch][WebGPU] Request shader-f16 device feature (enablement, fail-open) #20729
• [ExecuTorch][WebGPU] Unit tests for the WGSL shader-variant codegen #20728
• [ExecuTorch][WebGPU] WGSL shader-variant codegen (vec1/vec4, fp16-ready) + rms_norm dedup #20727

Locks the group_size % BK gate that selects the packed-word-dequant (pwdq) f16 steel GEMM. pwdq is bit-exact to the half kernel, so the existing steel_f16/steel_f16_edge configs (gs=32) already exercise it; these add the two group sizes the gate keys on but those omit.

Key changes:

• test_quantized_linear.py / test_webgpu_native.cpp — add pwdq_gs64 (gs=64, % BK == 0 → stays on pwdq) and pwdq_gs8 (gs=8 < BK=16 → falls back to the per-nibble half kernel, whose per-K scale read is correct there). Both goldened at the f16 tol (2.3e-4) in the WGPU_BACKEND_STEEL_F16 build against the fp64 dequant-matmul truth.

Co-authored-with: Claude Code.
@exported-using-ghexport

Differential Revision: D111163551

Differential Revision: D111163551

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❌ 4 New Failures, 12 Pending, 1 Unrelated Failure

As of commit fc07e9a with merge base f4b01a8 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/cortex_m/quantizer/pattern_matcher.py:
• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 22f3f74fd863dbfd3a460df5431b4ee861e42c2054fde36cb492bc53acc30fd7 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t fc56a655d1a7bf9da8770a39800f715b44499c23f50f6db8319cf8aa42f923e4 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t a8e730c5851d8abe12a6ff9ebe57542c7d6ee931edadb72f8fbe719f6df5f561 /exec failed with exit code 1

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / android / build-android (gh) (trunk failure)
  ImportError: /opt/hostedtoolcache/Python/3.11.15/x64/lib/python3.11/site-packages/executorch/extension/training/pybindings/_training_lib.cpython-311-x86_64-linux-gnu.so: undefined symbol: _ZN3c104impl3cow23materialize_cow_storageERNS_11StorageImplE

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