[ExecuTorch][WebGPU] Op-test codegen framework (cases.py -> generated .pte+golden -> gtest driver)

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add mul op with full broadcast (aten.mul.Tensor) #20358
• [ExecuTorch][WebGPU] Consolidate landed-op tests into the cases.py op-test framework #20357
• -> [ExecuTorch][WebGPU] Op-test codegen framework (cases.py -> generated .pte+golden -> gtest driver) #20339

A manifest-driven op-test framework for the WebGPU backend, mirroring Vulkan's op_tests/cases.py (declarative per-op suites) but with a torch-computed golden loaded in C++, since the native test binary has no ATen. An op fits when it is stateless and expressible as one module(inputs) -> golden forward; stateful KV-cache ops (sdpa, update_cache) stay hand-written. Lands first as the shared foundation for the following op test-diffs — adding an op's test becomes one cases.py entry.

Composition:

• test_suite.py — schema (WebGPUTestSuite/Case/InputSpec) + a register_op_test decorator; per-case required/heavy/golden_fn, per-suite golden_dtype.
• cases.py — the declarative suites; registers the landed add + rms_norm; later ops append one entry each.
• generate_op_tests.py — per case: export via VulkanPartitioner to .pte, compute the fp64 torch golden (dual-oracle gate), serialize inputs+golden as fp32, emit manifest.json.
• op_test_driver.cpp + driver_util.{h,cpp} — generic gtest driver: one test per manifest entry, runs forward on-device, abs/rel + shape + reconciliation checks.
• CMakeLists/ci.sh — webgpu_op_test + device-free webgpu_op_test_util_test, wired into the Dawn(Tint)+SwiftShader CI.
  @exported-using-ghexport

Differential Revision: D108816389

Differential Revision: D108816389

[pytorch-bot]

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❌ 5 New Failures, 6 Pending, 1 Unrelated Failure

As of commit b9f80dc with merge base eb7473b ():

NEW FAILURES - The following jobs have failed:

• Propose to merge ghstack orig PRs to main / Try to create a PR with ghstack /orig branch (gh)
  Process completed with exit code 1.
• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 0bf996049c9c70c571371d78eae25d4105a0f55ba92b4c6ad5ecbac4a561ce3a /exec failed with exit code 1
• pull / test-lora-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 80fdf68ea1151067ea4c1719ff87a17ab0af1bf6f65dcc38581358ba7c2e7971 /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7d2c3429af10f13e60b7238d18820eae812b7dd59cb5fed7c6a4972912b571ca /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch

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

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/m/mesa/mesa-vdpau-drivers_23.2.1-1ubuntu3.1%7e22.04.3_amd64.deb 404 Not Found [IP: 104.20.28.246 80]

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This pull request has been merged in 6f4c21e.