[ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_symint #555

Job	Run time
test-webgpu / test-backend-linux (webgpu, operators) / linux-job	45m 24s
test-webgpu / test-backend-linux (webgpu, models) / linux-job	26m 13s
test-webgpu / test-backend-macos	-1s
test-webgpu / package-golden-artifacts	1s
	1h 11m 37s

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