Function fft_batch 

pub fn fft_batch<T: Float>(
    input: &[Complex<T>],
    n: usize,
    howmany: usize,
) -> Vec<Complex<T>>

Convenience function for batched 1D forward FFT.

Performs howmany independent 1D FFTs, each of size n. Input and output are contiguous: batch i starts at index i * n.