**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
assert op1.dag is dag
        assert op1.owner == 'owner1'
        assert op2.dag is dag2
        assert op2.owner == 'owner2'
        with dag2:
            op3 = DummyOperator(task_id='op3')
        assert op3.dag is dag2
        assert op3.owner == 'owner2'
        with dag:
            with dag2:
                op4 = DummyOperator(task_id='op4')
            op5 = DummyOperator(task_id='op5')
        assert op4.dag is dag2
        assert op5.dag is dag
        assert op4.owner == 'owner2'
        assert op5.owner == 'owner1'
        with DAG('creating_dag_in_cm', start_date=DEFAULT_DATE) as dag:
            DummyOperator(task_id='op6')
        assert dag.dag_id == 'creating_dag_in_cm'
        assert dag.tasks[0].task_id == 'op6'
        with dag:
            with dag:
                op7 = DummyOperator(task_id='op7')
            op8 = DummyOperator(task_id='op8')
        op9 = DummyOperator(task_id='op8')
        op9.dag = dag2
        assert op7.dag == dag
        assert op8.dag == dag
        assert op9.dag == dag2
    def test_dag_topological_sort_include_subdag_tasks(self):