**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/serialization/serialized_objects.py
cls,
        encoded_group: Dict[str, Any],
        parent_group: Optional[TaskGroup],
        task_dict: Dict[str, BaseOperator],
    ) -> Optional[TaskGroup]:
        """Deserializes a TaskGroup from a JSON object."""
        if not encoded_group:
            return None
        group_id = cls._deserialize(encoded_group["_group_id"])
        kwargs = {
            key: cls._deserialize(encoded_group[key])
            for key in ["prefix_group_id", "tooltip", "ui_color", "ui_fgcolor"]
        }
        group = SerializedTaskGroup(group_id=group_id, parent_group=parent_group, **kwargs)
        group.children = {
            label: task_dict[val]
            if _type == DAT.OP  
            else SerializedTaskGroup.deserialize_task_group(val, group, task_dict)
            for label, (_type, val) in encoded_group["children"].items()
        }
        group.upstream_group_ids = set(cls._deserialize(encoded_group["upstream_group_ids"]))
        group.downstream_group_ids = set(cls._deserialize(encoded_group["downstream_group_ids"]))
        group.upstream_task_ids = set(cls._deserialize(encoded_group["upstream_task_ids"]))
        group.downstream_task_ids = set(cls._deserialize(encoded_group["downstream_task_ids"]))
        return group