Function rows_matvec 

pub fn rows_matvec(
    model: &Arc<CmfModel>,
    gw: &GraphW<'_>,
    rows: usize,
    cols: usize,
    x: &[f32],
    n: usize,
    y: &mut [f32],
) -> bool

y [n][rows] = W x [n][cols] for a graph weight on the selected device; false leaves the caller to run it (CUDA’s hybrid arm only).