**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/serialization/test_dag_serialization.py
"_downstream_task_ids": [],
                "_inlets": [],
                "_outlets": [],
                "ui_color": "#f0ede4",
                "ui_fgcolor": "#000",
                "template_fields": ['bash_command', 'env'],
                "template_fields_renderers": {'bash_command': 'bash', 'env': 'json'},
                "bash_command": "echo {{ task.task_id }}",
                'label': 'bash_task',
                "_task_type": "BashOperator",
                "_task_module": "airflow.operators.bash",
                "pool": "default_pool",
                "executor_config": {
                    '__type': 'dict',
                    '__var': {
                        "pod_override": {
                            '__type': 'k8s.V1Pod',
                            '__var': PodGenerator.serialize_pod(executor_config_pod),
                        }
                    },
                },
            },
            {
                "task_id": "custom_task",
                "retries": 1,
                "retry_delay": 300.0,
                "_downstream_task_ids": [],
                "_inlets": [],
                "_outlets": [],
                "_operator_extra_links": [{"tests.test_utils.mock_operators.CustomOpLink": {}}],