Support for kokkos-fft

[junghans]

Can we get support for kokkos-fft as a backend? This might make our live easier for Cabana down the road.

[mkstoyanov]

Recently has a meeting with a large part of the Kokkos team and discusses adopting heffte as the engine behind kokkos-mpi-fft (or whatever name they come up with). There was general interest, but I don't think there's enough strong will to make it happen.

On heFFTe side, making kokkos as a "backend" makes no sense. Kokkos is already a wrapper around vendor libraries like FFTW and cuFFT, and heFFTe talks to those directly. Adding an extra layer of indirection makes no sense.

Better kokkos-heFFTe integration can be achieved with some macros and templates, so we can infer the heFFTe backend from Kokkos and then have the ability to feed kokkos views directly into heFFTe. Should be simple, but I have not dabbled into Kokkos and would require some learning, and I have no funding to work on it (at the moment).

If someone familiar with Kokkos wants to contribute, I will help on this end.