[FEA] Implement `hostdevice_2dspan`

Similar to `hostdevice_span`, which can support viewing of a `hostdevice_vector`, we also need to have `hostdevice_2dspan` to view a `hostdevice_2dvector`. Having this will offer more flexibility especially in cuIO where 2d vectors are used a lot.

Note that some most important features that `hostdevice_2dspan` must have is the ability to transfer data between hot/device.