[ExecuTorch][WebGPU] Enable backend test suite + x86 CI

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][WebGPU] Enable backend test suite + x86 CI #19964
• [ExecuTorch][WebGPU] Add rms_norm op #19963

Wires the WebGPU backend into the standard ExecuTorch backend test suite and adds an x86 Linux CI job, mirroring the Vulkan delegate: backends/test/suite/flows/webgpu.py plus a WebGPUTester, run by oss/.github/workflows/test-backend-webgpu.yml on SwiftShader (a software Vulkan adapter, via wgpu-native, minimal dependencies, no GPU).

Two fixes were needed for SwiftShader's downlevel limits: request the adapter's full requiredLimits at device creation (software adapters default storage-buffer limits to 0), and make the add op's workgroup size dynamic instead of a hardcoded constant. The WGSL now declares a pipeline-overridable override wg_size: u32 = 256 and the host clamps it to the device's maxComputeInvocationsPerWorkgroup (256 on real GPUs and lavapipe, 128 on SwiftShader), so SwiftShader's 128-invocation cap no longer forces a smaller workgroup size on real hardware. This mirrors the dynamic-workgroup-sizing approach in D107259348 and opens the door to selecting device/algorithm-optimal sizes later. The add op also validates its 1D dispatch count before allocating any GPU objects, against the device's queried maxComputeWorkgroupsPerDimension (falling back to the WebGPU spec-default floor of 65535 only when the limit query fails). Per Stephen's review, the workgroup-size clamp and the dispatch-count computation are factored into reusable inline helpers in runtime/WebGPUUtils.h (clamp_workgroup_size and compute_1d_workgroup_count, mirroring the Vulkan delegate's utils::div_up) so the other ops can share them rather than re-inlining the logic. The editable CMake build additionally marks the vulkan_schema subdirectory EXCLUDE_FROM_ALL so the WebGPU ALL build does not pull in targets that need glslc.
@exported-using-ghexport

Differential Revision: D107288999

Differential Revision: D107288999

[pytorch-bot]

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❌ 3 New Failures, 3 Cancelled Jobs, 2 Unrelated Failures, 2 Unclassified Failures

As of commit a464b37 with merge base 915a82d ():

NEW FAILURES - The following jobs have failed:

• pull / test-static-llama-qnn-linux (stories_110m) / linux-job (gh)
  RuntimeError: Command docker exec -t 8d05ef7faeedee281947509fe1d6106e72a6ac4592feed0cbcd672399bfa865a /exec failed with exit code 92
• trunk / test-arm-backend-vkml (test_pytest_models_vkml) / linux-job (gh)
  RuntimeError: Command docker exec -t e1be6c697f4408ac9aa77afbfc67ff4d6043c250f8056f37132187a62f3adc40 /exec failed with exit code 1
• trunk / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t bd76c7071c20618d480affed120a8bc51971940bf87349e2908589b50d951741 /exec failed with exit code 92

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, models) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t 442766bd488c5015faf143e61aebf0770f2ae043585c425d7a6992c8ede80664 /exec failed with exit code 1
• Test WebGPU Backend / test-webgpu / test-backend-linux (webgpu, operators) / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command docker exec -t fb6630b05dad0262721629498a63d0b5d2acd83e12771c79ef86048166afea15 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / unittest / macos / macos-job (gh)
• pull / unittest-editable / macos / macos-job (gh)
• trunk / test-arm-backend-ethos-u (test_smaller_stories_llama) / linux-job (gh)

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)

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

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

• trunk / unittest-release / macos / macos-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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