Function spmv_buffer_size 

pub fn spmv_buffer_size<T: SparseScalar>(
    handle: &Handle,
    op: Op,
    alpha: &T,
    a: &SpMat<'_, T>,
    x: &DnVec<'_, T>,
    beta: &T,
    y: &DnVec<'_, T>,
    alg: SpMVAlg,
) -> Result<usize>

Query buffer-size for y = alpha * op(A) * x + beta * y.