[ExecuTorch][WebGPU] Add mul op with full broadcast (aten.mul.Tensor)

[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

Adds aten.mul.Tensor to the WebGPU delegate with full PyTorch broadcast, plus the shared runtime/ops/TensorMeta.h per-tensor uniform that broadcast ops reuse. Mul is on the Llama critical path — F.silu decomposes to sigmoid + mul, and SwiGLU multiplies two same-shape activations (the fast path).

Composition (single dispatch):

• TensorMeta.h (NEW) — 48-byte std140 {ndim, numel, sizes[4], strides[4]} UBO mirroring Vulkan's per-tensor BufferMetadata; fill_tensor_meta_broadcast right-aligns operand dims (rank>4 throws); static_assert(sizeof==48).
• mul/BinaryOp.cpp — builds 3 TensorMeta UBOs (out/in1/in2 at bindings 3/4/5), guards fp32 + rank≤4, 1D-dispatches over compute_1d_workgroup_count(numel), releases all uniforms after the bind group.
• mul/binary_mul.wgsl — same-shape fast path + a broadcast path (delinearize output index, clamp each input coord per-dim to size-1, relinearize on input strides).
• WebGPUUtils.h — adds the shared utils::make_uniform helper (first use).
  @exported-using-ghexport

Differential Revision: D108793167

Differential Revision: D108793167

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20358

• 📄 Preview Python docs built from this PR

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

❌ 5 New Failures, 6 Pending, 2 Unrelated Failures

As of commit b2df1c9 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 / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 1
• pull / test-arm-backend-no-driver (test_pytest_models_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 87e3eca68f37b25d53b605730ad71a24ca258f68017fc4a66d0bedb53d80627d /exec failed with exit code 1
• pull / test-voxtral-realtime-xnnpack-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e316b415060de10bea4d15d193523435e3e7b2d8978a407d27b21b73068296c3 /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 jobs failed but were 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]
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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.

[meta-codesync]

This pull request has been merged in 0e65ba6.