linear-algebra

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.

I noticed that CooMatrix::push_matrix does not threshold or detect zero elements in the input. Thus the resulting sparse matrix is more dense than it needs to be. By comparison, constructing a matrix using, e.g., CscMatrix::from, does detect entries that are exactly zero and does not include them in the resulting sparse matrix. If there isn't a rationale for the difference in behavior, may I s