expose impl ElemTypeOf for NotNan

[jacobrosenthal]

Amazing work.

I ran into the same Ord on float problem with a similar solution to yours

        let index = output_tensor
            .as_data::<NotNan<f32>>()
            .iter()
            .position_max()
            .unwrap();

But it doesnt look like you expose your impl ElemTypeOf for NotNan. Can we pub something to expose that?

Thanks!