Function sparse_fft2 

pub fn sparse_fft2<T>(
    signal: &[Vec<T>],
    k: usize,
    algorithm: Option<SparseFFTAlgorithm>,
) -> Result<SparseFFTResult, FFTError>
where
    T: NumCast + Copy + Debug + 'static,

2D sparse FFT

Computes a full 2D FFT (row-wise then column-wise) and returns the top-k frequency components by magnitude with their linear (row-major) indices.