[codex] Add WebGPU sigmoid operator

[iamorlando]

Summary

Adds WebGPU runtime support for aten.sigmoid.default, including:

• WGSL shader and generated embedded header
• C++ runtime op registration and CMake wiring
• WebGPU tester allowlist and export/op-test coverage
• README/TODO updates for current WebGPU Llama-oriented operator support

Test plan

• .venv/bin/python backends/webgpu/scripts/gen_wgsl_headers.py --check
• git diff HEAD --check
• PYTHONPYCACHEPREFIX=/tmp/executorch_pycache .venv/bin/python -m py_compile backends/webgpu/test/tester.py backends/webgpu/test/op_tests/cases.py backends/webgpu/test/op_tests/generate_op_tests.py backends/webgpu/test/op_tests/test_generator.py backends/webgpu/test/op_tests/test_schema.py backends/webgpu/test/ops/sigmoid/test_sigmoid.py
• PATH=/Users/orlando/Documents/dev/executorch/.venv/bin:$PATH .venv/bin/lintrunner --force-color <changed files> --skip MYPY (no lint issues)
• .venv/bin/python -m pytest backends/webgpu/test/ops/sigmoid/test_sigmoid.py -vv (4 passed)
• .venv/bin/python -m pytest backends/webgpu/test/op_tests/test_schema.py backends/webgpu/test/op_tests/test_generator.py -vv (11 passed)
• .venv/bin/python -m executorch.backends.webgpu.test.op_tests.generate_op_tests --output /tmp/webgpu_op_tests_all
• Linux amd64 Docker native build with Dawn prebuilt and Mesa lavapipe: cmake -DEXECUTORCH_BUILD_WEBGPU=ON -DEXECUTORCH_BUILD_WEBGPU_TEST=ON -DEXECUTORCH_BUILD_TESTS=ON ..., then cmake --build /tmp/build --target webgpu_scratch_buffer_test webgpu_op_test_util_test webgpu_op_test -j4
• /tmp/build/backends/webgpu/webgpu_scratch_buffer_test (passed)
• /tmp/build/backends/webgpu/webgpu_op_test_util_test (3 passed)
• /tmp/build/backends/webgpu/webgpu_op_test --manifest /artifacts/manifest.json (25 passed, including 5 OpTest_sigmoid cases)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20489

• 📄 Preview Python docs built from this PR

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❗ 1 Active SEVs

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• [ROCm] MI350 CI runner label rename: rebase PRs using old linux.rocm.gpu.gfx950.* labels

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