**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
t4.set_downstream(t6)
        t5.set_downstream(t6)
    :param tasks: List of tasks or List[airflow.models.BaseOperator] to set dependencies
    :type tasks: List[airflow.models.BaseOperator] or airflow.models.BaseOperator
    """
    for index, up_task in enumerate(tasks[:-1]):
        down_task = tasks[index + 1]
        if isinstance(up_task, BaseOperator):
            up_task.set_downstream(down_task)
            continue
        if isinstance(down_task, BaseOperator):
            down_task.set_upstream(up_task)
            continue
        if not isinstance(up_task, Sequence) or not isinstance(down_task, Sequence):
            raise TypeError(
                'Chain not supported between instances of {up_type} and {down_type}'.format(
                    up_type=type(up_task), down_type=type(down_task)
                )
            )
        up_task_list = up_task
        down_task_list = down_task
        if len(up_task_list) != len(down_task_list):
            raise AirflowException(
                f'Chain not supported different length Iterable '
                f'but get {len(up_task_list)} and {len(down_task_list)}'
            )
        for up_t, down_t in zip(up_task_list, down_task_list):
            up_t.set_downstream(down_t)
def cross_downstream(
    from_tasks: Sequence[BaseOperator], to_tasks: Union[BaseOperator, Sequence[BaseOperator]]