Right now we have and support "cpu" as a device for all supported libraries in the to_device() helper. In particular, we allow to_device(cupy_array, "cpu") to convert a CuPy array to a NumPy array.

This was implemented as a way to portably move an array to the host device, as per data-apis/array-api#626. However, it looks like that issue is going to be resolved in a different way, by adding a device keyword to from_dlpack data-apis/array-api#741.

So I propose we remove this behavior, as it extends things in a way that isn't specified by the standard. Since I'm not sure who is using this, we may want to issue a deprecation warning first.

CC @tylerjereddy @leofang