**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/dagrun.py
from airflow.configuration import conf as airflow_conf
from airflow.exceptions import AirflowException
from airflow.models.base import ID_LEN, Base
from airflow.models.taskinstance import TaskInstance as TI
from airflow.settings import task_instance_mutation_hook
from airflow.stats import Stats
from airflow.ti_deps.dep_context import DepContext
from airflow.ti_deps.dependencies_states import SCHEDULEABLE_STATES
from airflow.utils import callback_requests, timezone
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import UtcDateTime, nulls_first, skip_locked, with_row_locks
from airflow.utils.state import State
from airflow.utils.types import DagRunType
class TISchedulingDecision(NamedTuple):
    """Type of return for DagRun.task_instance_scheduling_decisions"""
    tis: List[TI]
    schedulable_tis: List[TI]
    changed_tis: bool
    unfinished_tasks: List[TI]
    finished_tasks: List[TI]
class DagRun(Base, LoggingMixin):
    """
    DagRun describes an instance of a Dag. It can be created
    by the scheduler (for regular runs) or by an external trigger
    """
    __tablename__ = "dag_run"
    id = Column(Integer, primary_key=True)
    dag_id = Column(String(ID_LEN))
    execution_date = Column(UtcDateTime, default=timezone.utcnow)