Iter2: store my output on the same device as the first input

[njaard]

In order for Iter2 to be usable with a GPU, you needed to do Iter2::new(...).to_device(vs.device()).shuffle()

Now with this patch, the to_device is not necessary, which is consistent with my expectations.

However, it has virtually no performance difference, positive or negative.

[vcarpani]

Thanks for this PR, what happens if xs and ys are on different devices?

Also, make sure to update the changelog adding an unreleased section.

[njaard]

@vcarpani It doesn't work, which is "philosophically" the same behavior as right now, but incompatible, because right now if the first parameter is GPU and the second is CPU, then Iter2 will work. With my patch. The first parameter must be the same as the second or you'll need to move the second it to the GPU.