**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
tests/dags/test_only_dummy_tasks.py
#
#
#
from datetime import datetime
from airflow.models import DAG
from airflow.operators.dummy_operator import DummyOperator
DEFAULT_DATE = datetime(2016, 1, 1)
default_args = {
    "owner": "airflow",
    "start_date": DEFAULT_DATE,
}
dag = DAG(dag_id="test_only_dummy_tasks", default_args=default_args, schedule_interval='@once')
with dag:
    task_a = DummyOperator(task_id="test_task_a")
    task_b = DummyOperator(task_id="test_task_b")
    task_a >> task_b
    task_c = DummyOperator(task_id="test_task_c")
    task_d = DummyOperator(task_id="test_task_on_execute", on_execute_callback=lambda *args, **kwargs: 1)
    task_e = DummyOperator(task_id="test_task_on_success", on_success_callback=lambda *args, **kwargs: 1)