Function q8_matvec_range 

pub fn q8_matvec_range(
    model: &Arc<CmfModel>,
    idx: usize,
    row0: usize,
    row_scale: &[f32],
    xs: &[f32],
    rows: usize,
    cols: usize,
    out: &mut [f32],
) -> bool

q8_row/q8_2f matvec, rows [row0, row0+rows). xs — prescaled by the θ-field.