[Vulkan] Delegate aten.mul.Scalar

[mergennachin]

Superseded by #23252, part 13/15 of the ghstack replacement series. The current integration PR is #23254. This PR is closed in favor of the replacement; its previous description and review history are retained.

---

Tensor-times-scalar products appear in attention scaling and were kept on CPU, splitting otherwise fully delegated blocks. This adds a mul variant to the binary scalar shaders and registers it with pow.Tensor_Scalar under a shared op_registry entry that applies the same scalar-value guard.

Part 13/15 of the Vulkan transformer and operator-conformance stack. Depends on #23212; review against the selected base branch. Integration PR: #23162.

Validation: 1 passed, 2 warnings in 10.21s. Lintrunner and git diff --check pass. Native tests use MoltenVK with portable CPU kernels; hardware and SwiftShader CI are pending.

Authored with OpenAI Codex; split planned with Claude Code.

cc @SS-JIA @manuelcandales @digantdesai @cbilgin

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/23213

• 📄 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

As of commit 35ddf0b with merge base a318382 ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / macos / macos-job (gh)
  examples/models/muse-glimmer/tests/test_vision_precompute.py::PrecomputeBundleTest::test_all_inputs_consistent

This comment was automatically generated by Dr. CI and updates every 15 minutes.