**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
assert serialized_dag.full_filepath == dag.fileloc
    def validate_deserialized_task(
        self,
        serialized_task,
        task,
    ):
        """Verify non-airflow operators are casted to BaseOperator."""
        assert isinstance(serialized_task, SerializedBaseOperator)
        assert not isinstance(task, SerializedBaseOperator)
        assert isinstance(task, BaseOperator)
        fields_to_check = task.get_serialized_fields() - {
            
            '_task_type',
            'subdag',
            
            '_log',
            
            'template_fields',
            
            'on_failure_callback',
            'on_success_callback',
            'on_retry_callback',
            
            'resources',
        }
        assert serialized_task.task_type == task.task_type
        assert set(serialized_task.template_fields) == set(task.template_fields)
        assert serialized_task.upstream_task_ids == task.upstream_task_ids
        assert serialized_task.downstream_task_ids == task.downstream_task_ids
        for field in fields_to_check: