**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
self,
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        session: Session = None,
    ) -> List[TaskInstance]:
        """
        Get a set of task instance related to this task for a specific date
        range.
        """
        end_date = end_date or timezone.utcnow()
        return (
            session.query(TaskInstance)
            .filter(TaskInstance.dag_id == self.dag_id)
            .filter(TaskInstance.task_id == self.task_id)
            .filter(TaskInstance.execution_date >= start_date)
            .filter(TaskInstance.execution_date <= end_date)
            .order_by(TaskInstance.execution_date)
            .all()
        )
    def get_flat_relative_ids(