ForwardDiff and Zygote cannot automatically differentiate (AD) function from C^n to R that uses FFT

[ChrisRackauckas]

@tholdem

The only problem left is fft not knowing how to handle ForwardDiff dual type.

Just do an overload to fft so you don’t send the duals around it but just multiply the result. Differentiating fft is too trivial to actually need to differentiate through the algorithm.