ggml : add FFT operator

[ggerganov]

It would be nice to have a simple implementation of FFT. The main goal is to be able to keep the data inside a ggml graph in certain scenarios (such as in Whisper and various TTS models), even if the performance is not as good as dedicated libraries (such as FFTW and cuFFT). I think we can start with a basic Cooley-Tukey implementation to keep things simple. Batched 1D FFT should be supported, as well as common variations such as inverse FFT and real-valued FFT.

[maedoc]

I'm interested in trying but don't see any complex number support in ggml so far (IIUC). Just curious if there's any thought so far about data layout (separate, interleaved etc). Seems like the easiest initial approach would be to just keep the real & imag parts separate and imag part optional for some ops in the API.