[ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_symint #637

Job	Run time
test-webgpu / test-backend-linux (webgpu, models) / linux-job	28m 7s
test-webgpu / test-backend-linux (webgpu, operators) / linux-job	49m 35s
test-webgpu / test-backend-macos	-8s
test-webgpu / package-golden-artifacts	1m 35s
	1h 19m 9s

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