Function reduce_axis 

pub fn reduce_axis<T: Copy + MaybeSendSync, Op: ElementOp<T>, M, R, U>(
    src: &StridedView<'_, T, Op>,
    axis: usize,
    map_fn: M,
    reduce_fn: R,
    init: U,
) -> Result<StridedArray<U>>
where
    M: Fn(T) -> U + MaybeSync,
    R: Fn(U, U) -> U + MaybeSync,
    U: Clone + MaybeSendSync,

Reduce along a single axis, returning a new StridedArray.

This allocates a column-major output and delegates to reduce_axis_into.

Every output folds init once and then its reduced elements. When the reduced axis has unit stride those elements are folded with the lane scheme of reduce, so reduce_fn must be associative and commutative. When the leading kept axis has unit stride, a block of adjacent outputs is accumulated one contiguous source column at a time, which keeps the left to right order per output. With the parallel feature, outputs are split across threads once the number of source elements read exceeds the threading threshold; each output is computed by one thread, so the result does not depend on the thread count.