Function spmm_buffer_size 

pub fn spmm_buffer_size<T: SparseScalar>(
    handle: &Handle,
    op_a: Op,
    op_b: Op,
    alpha: &T,
    a: &SpMat<'_, T>,
    b: &DnMat<'_, T>,
    beta: &T,
    c: &DnMat<'_, T>,
    alg: SpMMAlg,
) -> Result<usize>

Query buffer-size for C = alpha * op(A) * op(B) + beta * C, A sparse.