**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
dag = DAG(
            'dag',
            start_date=DEFAULT_DATE,
            default_view="graph",
        )
        with dag:
            DummyOperator(task_id='task', owner='owner1')
            SubDagOperator(
                task_id='subtask',
                owner='owner2',
                subdag=DAG(
                    'dag.subtask',
                    start_date=DEFAULT_DATE,
                ),
            )
        session = settings.Session()
        dag.sync_to_db(session=session)
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == 'dag').one()
        assert orm_dag.default_view is not None
        assert orm_dag.default_view == "graph"
        session.close()
    @provide_session
    def test_is_paused_subdag(self, session):