Use `fft` and `rfft` as fit

[pzimbrod]

#31 introduces Fourier Layer for higher dimensions. For now, rfft has been replaced by fft since it's simpler to implement.

However, rfft can save memory and computation time considerably (~2x), so it should be picked up again.

I think #28 plays a role here as well. If the input is complex, then rfft doesn't make sense. But since we're working with @generated anyway, maybe we can include a check where we can decide whether to use regular or real FFT depending on the input.