**Description**

It would be nice if the weight_rule default value could be managed by a global config

suggested config:
```
# Weighting method used for the effective total priority weight of the task. 
# Options are: { downstream | upstream | absolute } default is
default_weight_rule = downstream
```
**Use case / motivation**

In some pipeline, you really need to have absolute weight, and then you have to add a line in each task definition which is annoying

tests/models/test_baseoperator.py
def test_chain(self):
        dag = DAG(dag_id='test_chain', start_date=datetime.now())
        
        [label1, label2] = [Label(label=f"label{i}") for i in range(1, 3)]
        [op1, op2, op3, op4, op5, op6] = [BaseOperator(task_id=f't{i}', dag=dag) for i in range(1, 7)]
        chain(op1, [label1, label2], [op2, op3], [op4, op5], op6)
        assert {op2, op3} == set(op1.get_direct_relatives(upstream=False))
        assert [op4] == op2.get_direct_relatives(upstream=False)
        assert [op5] == op3.get_direct_relatives(upstream=False)
        assert {op4, op5} == set(op6.get_direct_relatives(upstream=True))
        assert {"label": "label1"} == dag.get_edge_info(
            upstream_task_id=op1.task_id, downstream_task_id=op2.task_id
        )
        assert {"label": "label2"} == dag.get_edge_info(
            upstream_task_id=op1.task_id, downstream_task_id=op3.task_id
        )
        
        [xlabel1, xlabel2] = [Label(label=f"xcomarg_label{i}") for i in range(1, 3)]
        [xop1, xop2, xop3, xop4, xop5, xop6] = [
            task_decorator(task_id=f"xcomarg_task{i}", python_callable=lambda: None, dag=dag)()
            for i in range(1, 7)
        ]
        chain(xop1, [xlabel1, xlabel2], [xop2, xop3], [xop4, xop5], xop6)
        assert {xop2.operator, xop3.operator} == set(xop1.operator.get_direct_relatives(upstream=False))
        assert [xop4.operator] == xop2.operator.get_direct_relatives(upstream=False)
        assert [xop5.operator] == xop3.operator.get_direct_relatives(upstream=False)
        assert {xop4.operator, xop5.operator} == set(xop6.operator.get_direct_relatives(upstream=True))
        assert {"label": "xcomarg_label1"} == dag.get_edge_info(
            upstream_task_id=xop1.operator.task_id, downstream_task_id=xop2.operator.task_id
        )