[ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_s… #13930

Job	Run time
CI run decision / decide	33s
Get changed files / get-changed-files	3s
export-model-cuda-artifact	-1s
test-executorch-cuda-build-${{ matrix.cuda-version }}	-1s
test-models-cuda	-1s
check-all-cuda-builds	-1s
unittest-cuda	-1s
test-cuda-pybind	-1s
test-model-cuda-e2e	-1s
	29s

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