**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
super().__setattr__(key, value)
        if self._lock_for_execution:
            
            return
        if self.__instantiated and key in self.template_fields:
            
            
            
            
            self.set_xcomargs_dependencies()
    def add_inlets(self, inlets: Iterable[Any]):
        """Sets inlets to this operator"""
        self._inlets.extend(inlets)
    def add_outlets(self, outlets: Iterable[Any]):
        """Defines the outlets of this operator"""
        self._outlets.extend(outlets)
    def get_inlet_defs(self):
        """:return: list of inlets defined for this operator"""
        return self._inlets
    def get_outlet_defs(self):