**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
child_dag = DAG(
            'parent_dag.child_dag',
            schedule_interval='@daily',
            start_date=DEFAULT_DATE,
        )
        with child_dag:
            DummyOperator(task_id='a_child')
            DummyOperator(task_id='b_child')
        parent_dag = DAG(
            'parent_dag',
            schedule_interval='@daily',
            start_date=DEFAULT_DATE,
        )
        
        with parent_dag:
            op1 = DummyOperator(task_id='a_parent')
            op2 = SubDagOperator(task_id='child_dag', subdag=child_dag)
            op3 = DummyOperator(task_id='b_parent')
            op1 >> op2 >> op3
        topological_list = parent_dag.topological_sort(include_subdag_tasks=True)
        assert self._occur_before('a_parent', 'child_dag', topological_list)
        assert self._occur_before('child_dag', 'a_child', topological_list)
        assert self._occur_before('child_dag', 'b_child', topological_list)
        assert self._occur_before('a_child', 'b_parent', topological_list)
        assert self._occur_before('b_child', 'b_parent', topological_list)
    def test_dag_topological_sort1(self):