**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("test_schedule_interval", schedule_interval=schedule_interval)
        assert dag.normalized_schedule_interval == expected_n_schedule_interval
        assert dag.schedule_interval == schedule_interval
    def test_set_dag_runs_state(self):
        clear_db_runs()
        dag_id = "test_set_dag_runs_state"
        dag = DAG(dag_id=dag_id)
        for i in range(3):
            dag.create_dagrun(run_id=f"test{i}", state=State.RUNNING)
        dag.set_dag_runs_state(state=State.NONE)
        drs = DagRun.find(dag_id=dag_id)
        assert len(drs) == 3
        assert all(dr.state == State.NONE for dr in drs)
    def test_create_dagrun_run_id_is_generated(self):
        dag = DAG(dag_id="run_id_is_generated")
        dr = dag.create_dagrun(run_type=DagRunType.MANUAL, execution_date=DEFAULT_DATE, state=State.NONE)
        assert dr.run_id == f"manual__{DEFAULT_DATE.isoformat()}"
    def test_create_dagrun_run_type_is_obtained_from_run_id(self):
        dag = DAG(dag_id="run_type_is_obtained_from_run_id")
        dr = dag.create_dagrun(run_id="scheduled__", state=State.NONE)
        assert dr.run_type == DagRunType.SCHEDULED
        dr = dag.create_dagrun(run_id="custom_is_set_to_manual", state=State.NONE)
        assert dr.run_type == DagRunType.MANUAL
    def test_create_dagrun_job_id_is_set(self):