**Apache Airflow version**:
v2.0.0

**What happened**:

Custom operators inheriting from DummyOperator are go in to a scheduled state, where as DummyOperator itself is set straight to success.

**What you expected to happen**:

All operators should be successfully executed.

**How to reproduce it**:

```py
from datetime import datetime
from airflow import DAG
from airflow.operators.dummy_operator import DummyOperator

class MyJob(DummyOperator):
    template_fields_renderers = {
        "body": "json"
    }
    template_fields = ("body",)

    def __init__(self, body, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.body = body

with DAG(
    dag_id="test",
    schedule_interval=None,
    start_date=datetime(2020, 8, 13),
) as dag:
    MyJob(task_id="aaaa", body={"aaa": "bbb"})

```

**Anything else we need to know**:
This is a known problem that we will fix, see: https://github.com/apache/airflow/pull/10956#discussion_r502492775
airflow/models/baseoperator.py
dags = {
            task._dag.dag_id: task._dag  
            for task in self.roots + task_list
            if task.has_dag()  
        }
        if len(dags) > 1:
            raise AirflowException(
                f'Tried to set relationships between tasks in more than one DAG: {dags.values()}'
            )
        elif len(dags) == 1:
            dag = dags.popitem()[1]
        else:
            raise AirflowException(
                "Tried to create relationships between tasks that don't have "
                "DAGs yet. Set the DAG for at least one "
                "task  and try again: {}".format([self] + task_list)
            )
        if dag and not self.has_dag():
            self.dag = dag
        for task in task_list:
            if dag and not task.has_dag():
                task.dag = dag
            if upstream:
                task.add_only_new(task.get_direct_relative_ids(upstream=False), self.task_id)
                self.add_only_new(self._upstream_task_ids, task.task_id)
            else:
                self.add_only_new(self._downstream_task_ids, task.task_id)
                task.add_only_new(task.get_direct_relative_ids(upstream=True), self.task_id)
    def set_downstream(self, task_or_task_list: Union[TaskMixin, Sequence[TaskMixin]]) -> None: