**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
"""Required by TaskMixin"""
        return [self]
    @property
    def leaves(self) -> List["BaseOperator"]:
        """Required by TaskMixin"""
        return [self]
    def _set_relatives(
        self,
        task_or_task_list: Union[TaskMixin, Sequence[TaskMixin]],
        upstream: bool = False,
    ) -> None:
        """Sets relatives for the task or task list."""
        if not isinstance(task_or_task_list, Sequence):
            task_or_task_list = [task_or_task_list]
        task_list: List["BaseOperator"] = []
        for task_object in task_or_task_list:
            task_object.update_relative(self, not upstream)
            relatives = task_object.leaves if upstream else task_object.roots
            task_list.extend(relatives)
        for task in task_list:
            if not isinstance(task, BaseOperator):
                raise AirflowException(
                    "Relationships can only be set between "
                    "Operators; received {}".format(task.__class__.__name__)
                )