At this moment relu_layer op doesn't allow threshold configuration, and legacy RELU op allows that.
We should add configuration option to relu_layer.

Add quickselect algorithm. Quickselect is a selection algorithm to find the kth smallest element in an unordered list.

I am trying to move a matrix into an iterator over its rows or columns. row_iter() borrows the matrix, which breaks my usecase:

fn large_f<N, It: Iterator<Item=N>>(matrix_map: impl Fn(Matrix) -> It, mat: Matrix) { ... }

I need a a function that can map a matrix to an iterator for the same operations either on the matrix' rows or columns.

let mat = ...;
large_f(
    |mat