**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
"""Returns True if the Operator has been assigned to a DAG."""
        return getattr(self, '_dag', None) is not None
    @property
    def dag_id(self) -> str:
        """Returns dag id if it has one or an adhoc + owner"""
        if self.has_dag():
            return self.dag.dag_id
        else:
            return 'adhoc_' + self.owner
    @property
    def deps(self) -> Set[BaseTIDep]:
        """
        Returns the set of dependencies for the operator. These differ from execution
        context dependencies in that they are specific to tasks and can be
        extended/overridden by subclasses.
        """
        return {
            NotInRetryPeriodDep(),
            PrevDagrunDep(),
            TriggerRuleDep(),
            NotPreviouslySkippedDep(),
        }
    def prepare_for_execution(self) -> "BaseOperator":