Function matmul 

pub fn matmul<T, D, RA, RB, SA, SB>(
    a: &Matrix<RA, SA, D>,
    b: &Matrix<RB, SB, D>,
) -> Matrix<Owned<T>, DimDyn, D>
where
    T: Num,
    D: Device,
    RA: Repr<Item = T>,
    RB: Repr<Item = T>,
    SA: DimTrait,
    SB: DimTrait,