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

[JCNTH]

Stack from ghstack (oldest at bottom):

• (to be filled)

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.

Differential Revision: D110919973

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20771

• 📄 Preview Python docs built from this PR

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

❌ 1 New Failure, 1 Cancelled Job

As of commit b2d3cef with merge base f8c8334 ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 760d86510813bd44f842b770bcdc944945a060e7fe416526ef38a212f59ea2e5 /exec failed with exit code 139

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.