**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
def setUp(self) -> None:
        clear_db_runs()
    def tearDown(self) -> None:
        clear_db_runs()
    @parameterized.expand(
        [
            (3,),
            (12,),
        ]
    )
    def test_count_number_queries(self, tasks_count):
        dag = DAG('test_dagrun_query_count', start_date=DEFAULT_DATE)
        for i in range(tasks_count):
            DummyOperator(task_id=f'dummy_task_{i}', owner='test', dag=dag)
        with assert_queries_count(2):
            dag.create_dagrun(
                run_id="test_dagrun_query_count",
                state=State.RUNNING,
                execution_date=TEST_DATE,
            )
class TestDagDecorator(unittest.TestCase):