[ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_symint #38860

Job	Run time
gather-models	10s
test-models-linux (cmake, vit, portable, linux.2xlarge, 90) / linux-job	16m 43s
test-models-linux (cmake, mv3, portable, linux.2xlarge, 90) / linux-job	9m 35s
test-models-linux (cmake, vit, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job	10m 44s
test-models-linux (buck2, mv3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job	10m 35s
test-models-linux (cmake, mv3, xnnpack-quantization-delegation, linux.2xlarge, 90) / linux-job	9m 44s
test-models-linux (buck2, mv3, portable, linux.2xlarge, 90) / linux-job	9m 31s
	1h 7m 2s

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