Request for argMax op

[Honry]

argMax has been supported in both TensorFlow and ONNX, and which is an important op in DeepLabV3 models, so it'd be great if WebNN could support this op as well.

[wchao1115]

and so should argMin.

[inexorabletash]

Can this be closed now that PR #478 has merged?

[fdwr]

Yep, should be closed now that argMin and argMax are in.