[ExecuTorch][WebGPU] Test coverage for the f16 KV cache

[JCNTH]

Stack from ghstack (oldest at bottom):

• -> [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

Tests for the opt-in f16 KV cache (stacked op diff). When built with EXECUTORCH_WEBGPU_KV_F16 and run on a shader-f16 device, the SDPA op stores the K/V cache as f16, so the entire existing sdpa_with_kv_cache golden suite (config sweep + replay + dynamic decode) exercises the f16-KV path with no new goldens needed; this diff loosens the SDPA numeric tolerance to the f16 read-precision floor when — and only when — that path is active.

Key changes:

• test_webgpu_native.cpp sdpa_within_tol — under #ifdef WGPU_BACKEND_KV_F16, compare at abs 2e-3 / rel 1e-2 when the device negotiated shader-f16, else keep the strict f32 abs 1e-4 / rel 1e-3. Every SDPA config/replay/decode golden then validates the f16-KV output through the existing exemplars.

Constraints: the default (flag-OFF) test build is unchanged; on a non-shader-f16 device (including the CI software adapter) the f16 KV path stays inactive and the strict f32 tolerance applies, so there is no CI behavior change. The f16-KV numeric validation is therefore shader-f16-device-only (Canary/Metal), matching the op's opt-in gating; no CI script change (mirrors the steel-f16 tests).

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

Differential Revision: D110919973

Differential Revision: D110919973

[pytorch-bot]

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❌ 1 New Failure, 1 Pending, 1 Unrelated Failure

As of commit 1479030 with merge base f8c8334 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  The process '/usr/local/lib/android/sdk/platform-tools/adb' failed with exit code 224

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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