**Apache Airflow version**: 2.0.0

**Environment**:

- **Cloud provider or hardware configuration**: Docker container
- **OS** (e.g. from /etc/os-release): Debian Stretch

**What happened**:

When working on some custom DAG override logic, I noticed that invoking `DAG.partial_subset` does not properly update the corresponding `_task_group.used_group_ids` on the returned subset DAG, such that adding back a task which was excluded during the `partial_subset` operation fails.

**What you expected to happen**:

Tasks that had already been added to the original DAG can be added again to the subset DAG returned by `DAG.partial_subset`

**How to reproduce it**:
Create any DAG with a single task called, e.g. `my-task`, then invoke `dag.partial_subset(['not-my-task'], False, False)`

Note that the returned subset DAG's `_task_group.used_group_ids` still contains `my-task` even though it was not included in the subset DAG itself

**Anything else we need to know**:

I was able to work around this by adding logic to update the new partial subset DAG's `_task_group.used_group_ids` manually, but this should really be done as part of the `DAG.partial_subset` logic
tests/models/test_dag.py
job_id = 42
        dag = DAG(dag_id="test_create_dagrun_job_id_is_set")
        dr = dag.create_dagrun(
            run_id="test_create_dagrun_job_id_is_set", state=State.NONE, creating_job_id=job_id
        )
        assert dr.creating_job_id == job_id
    @parameterized.expand(
        [
            (State.NONE,),
            (State.RUNNING,),
        ]
    )
    def test_clear_set_dagrun_state(self, dag_run_state):
        dag_id = 'test_clear_set_dagrun_state'
        self._clean_up(dag_id)
        task_id = 't1'
        dag = DAG(dag_id, start_date=DEFAULT_DATE, max_active_runs=1)
        t_1 = DummyOperator(task_id=task_id, dag=dag)
        session = settings.Session()
        dagrun_1 = dag.create_dagrun(
            run_type=DagRunType.BACKFILL_JOB,
            state=State.FAILED,
            start_date=DEFAULT_DATE,
            execution_date=DEFAULT_DATE,
        )
        session.merge(dagrun_1)