'Sum the linear expressions.
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):

    for (i, arg) in enumerate(arg_objs):
        if (arg.size != size):
            arg_objs[i] = lu.promote(arg, size)
    return (lu.sum_expr(arg_objs), [])
