'Sum the linear expression\'s entries.
Parameters
arg_objs : list
LinExpr for each argument.
size : tuple
The size of the resulting expression.
data :
Additional data required by the atom.
Returns
tuple
(LinOp for objective, list of constraints)'

@staticmethod
def graph_implementation(arg_objs, size, data=None):

    axis = data[0]
    if (axis is None):
        obj = lu.sum_entries(arg_objs[0])
    elif (axis == 1):
        const_size = (arg_objs[0].size[1], 1)
        ones = lu.create_const(np.ones(const_size), const_size)
        obj = lu.rmul_expr(arg_objs[0], ones, size)
    else:
        const_size = (1, arg_objs[0].size[0])
        ones = lu.create_const(np.ones(const_size), const_size)
        obj = lu.mul_expr(ones, arg_objs[0], size)
    return (obj, [])
