Function chunked_sum 

fn chunked_sum<T>(
    chunks: usize,
    range: impl Fn(usize) -> Range<usize> + Sync,
    f: impl Fn(usize) -> T + Sync,
) -> T
where
    T: Copy + Send + Sync + Add<Output = T> + Default,

f summed over each of chunks ranges in order, then the partial sums in order