**Apache Airflow version: 2.0.1**
**What happened:**
Running an airflow dags test or backfill  CLI command shown in tutorial, produces the same error.

**dags test cli command result：**

```
(airflow_venv) (base) app@lunar_01:airflow$ airflow dags test tutorial 2015-06-01
[2021-02-16 04:29:22,355] {dagbag.py:448} INFO - Filling up the DagBag from /home/app/Lunar/src/airflow/dags
[2021-02-16 04:29:22,372] {example_kubernetes_executor_config.py:174} WARNING - Could not import DAGs in example_kubernetes_executor_config.py: No module named 'kubernetes'
[2021-02-16 04:29:22,373] {example_kubernetes_executor_config.py:175} WARNING - Install kubernetes dependencies with: pip install apache-airflow['cncf.kubernetes']
Traceback (most recent call last):
  File "/home/app/airflow_venv/bin/airflow", line 10, in <module>
    sys.exit(main())
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/cli/cli_parser.py", line 48, in command
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/cli.py", line 89, in wrapper
    return f(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/cli/commands/dag_command.py", line 389, in dag_test
    dag.run(executor=DebugExecutor(), start_date=args.execution_date, end_date=args.execution_date)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/models/dag.py", line 1706, in run
    job.run()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/base_job.py", line 237, in run
    self._execute()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 805, in _execute
    session=session,
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 715, in _execute_for_run_dates
    tis_map = self._task_instances_for_dag_run(dag_run, session=session)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 359, in _task_instances_for_dag_run
    dag_run.refresh_from_db()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/models/dagrun.py", line 178, in refresh_from_db
    DR.run_id == self.run_id,
  File "/home/app/airflow_venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 3500, in one
    raise orm_exc.NoResultFound("No row was found for one()")
sqlalchemy.orm.exc.NoResultFound: No row was found for one()
```

**backfill cli command result:**
```
(airflow_venv) (base) app@lunar_01:airflow$ airflow dags backfill tutorial --start-date 2015-06-01 --end-date 2015-06-07
/home/app/airflow_venv/lib/python3.7/site-packages/airflow/cli/commands/dag_command.py:62 PendingDeprecationWarning: --ignore-first-depends-on-past is deprecated as the value is always set to True
[2021-02-16 04:30:16,979] {dagbag.py:448} INFO - Filling up the DagBag from /home/app/Lunar/src/airflow/dags
[2021-02-16 04:30:16,996] {example_kubernetes_executor_config.py:174} WARNING - Could not import DAGs in example_kubernetes_executor_config.py: No module named 'kubernetes'
[2021-02-16 04:30:16,996] {example_kubernetes_executor_config.py:175} WARNING - Install kubernetes dependencies with: pip install apache-airflow['cncf.kubernetes']
Traceback (most recent call last):
  File "/home/app/airflow_venv/bin/airflow", line 10, in <module>
    sys.exit(main())
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/cli/cli_parser.py", line 48, in command
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/cli.py", line 89, in wrapper
    return f(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/cli/commands/dag_command.py", line 116, in dag_backfill
    run_backwards=args.run_backwards,
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/models/dag.py", line 1706, in run
    job.run()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/base_job.py", line 237, in run
    self._execute()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 805, in _execute
    session=session,
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 715, in _execute_for_run_dates
    tis_map = self._task_instances_for_dag_run(dag_run, session=session)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 62, in wrapper
    return func(*args, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/jobs/backfill_job.py", line 359, in _task_instances_for_dag_run
    dag_run.refresh_from_db()
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/utils/session.py", line 65, in wrapper
    return func(*args, session=session, **kwargs)
  File "/home/app/airflow_venv/lib/python3.7/site-packages/airflow/models/dagrun.py", line 178, in refresh_from_db
    DR.run_id == self.run_id,
  File "/home/app/airflow_venv/lib/python3.7/site-packages/sqlalchemy/orm/query.py", line 3500, in one
    raise orm_exc.NoResultFound("No row was found for one()")
sqlalchemy.orm.exc.NoResultFound: No row was found for one()
```
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) -> Optional['DagRun']: