Function sparse_fftn 

pub fn sparse_fftn<T>(
    _signal: &[T],
    _shape: &[usize],
    _k: usize,
    _algorithm: Option<SparseFFTAlgorithm>,
) -> FFTResult<SparseFFTResult>
where
    T: NumCast + Copy + Debug + 'static,

N-dimensional sparse FFT (placeholder implementation)