**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
self.log.warning("Failed to get task '%s' for dag '%s'. Marking it as removed.", ti, dag)
                    Stats.incr(f"task_removed_from_dag.{dag.dag_id}", 1, 1)
                    ti.state = State.REMOVED
            should_restore_task = (task is not None) and ti.state == State.REMOVED
            if should_restore_task:
                self.log.info("Restoring task '%s' which was previously removed from DAG '%s'", ti, dag)
                Stats.incr(f"task_restored_to_dag.{dag.dag_id}", 1, 1)
                ti.state = State.NONE
            session.merge(ti)
        
        for task in dag.task_dict.values():
            if task.start_date > self.execution_date and not self.is_backfill:
                continue
            if task.task_id not in task_ids:
                Stats.incr(f"task_instance_created-{task.task_type}", 1, 1)
                ti = TI(task, self.execution_date)
                task_instance_mutation_hook(ti)
                session.add(ti)
        try:
            session.flush()
        except IntegrityError as err:
            self.log.info(str(err))
            self.log.info(
                'Hit IntegrityError while creating the TIs for ' f'{dag.dag_id} - {self.execution_date}.'
            )
            self.log.info('Doing session rollback.')
            
            session.rollback()
    @staticmethod
    def get_run(session: Session, dag_id: str, execution_date: datetime):