* VTK-m: Make vtk-m consistent with ROCmPackage
* VTKm: Add kokkos variant
Specifying +kokkos will enable kokkos backend.
Specifying +kokkos with +rocm will require a kokkos with a ROCm backend.
Specifying +cuda enables VTK-m native CUDA backend. VTK-m native cuda backend
is not compatible with the kokkos +cuda backend.
* VTK-m: Add cuda_native variant
Required to allow specifying a vtk-m spec the selects a
cuda_arch and predictably propagate that to the underlying kokkos
dependency.
This also makes explicit selecting kokkos with a cuda backend or using
the VTK-m cuda backend.
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