**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
with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = DummyOperator(task_id='t1')
            op2 = DummyOperator(task_id='t2')
            op3 = DummyOperator(task_id='t3')
            op1 >> op2
            op2 >> op3
        sub_dag = dag.sub_dag('t2', include_upstream=True, include_downstream=False)
        assert id(sub_dag.task_dict['t1'].downstream_list[0].dag) == id(sub_dag)
    def test_schedule_dag_no_previous_runs(self):
        """
        Tests scheduling a dag with no previous runs
        """
        dag_id = "test_schedule_dag_no_previous_runs"
        dag = DAG(dag_id=dag_id)
        dag.add_task(BaseOperator(task_id="faketastic", owner='Also fake', start_date=TEST_DATE))
        dag_run = dag.create_dagrun(
            run_type=DagRunType.SCHEDULED,
            execution_date=TEST_DATE,
            state=State.RUNNING,
        )
        assert dag_run is not None
        assert dag.dag_id == dag_run.dag_id
        assert dag_run.run_id is not None
        assert '' != dag_run.run_id
        assert (