**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
ti4 = TI(task=test_task, execution_date=DEFAULT_DATE + datetime.timedelta(days=3))
        ti4.state = State.RUNNING
        session = settings.Session()
        session.merge(ti1)
        session.merge(ti2)
        session.merge(ti3)
        session.merge(ti4)
        session.commit()
        assert 0 == DAG.get_num_task_instances(test_dag_id, ['fakename'], session=session)
        assert 4 == DAG.get_num_task_instances(test_dag_id, [test_task_id], session=session)
        assert 4 == DAG.get_num_task_instances(test_dag_id, ['fakename', test_task_id], session=session)
        assert 1 == DAG.get_num_task_instances(test_dag_id, [test_task_id], states=[None], session=session)
        assert 2 == DAG.get_num_task_instances(
            test_dag_id, [test_task_id], states=[State.RUNNING], session=session
        )
        assert 3 == DAG.get_num_task_instances(
            test_dag_id, [test_task_id], states=[None, State.RUNNING], session=session
        )
        assert 4 == DAG.get_num_task_instances(
            test_dag_id, [test_task_id], states=[None, State.QUEUED, State.RUNNING], session=session
        )
        session.close()
    def test_user_defined_filters(self):
        def jinja_udf(name):
            return f'Hello {name}'
        dag = models.DAG('test-dag', start_date=DEFAULT_DATE, user_defined_filters={"hello": jinja_udf})
        jinja_env = dag.get_template_env()
        assert 'hello' in jinja_env.filters
        assert jinja_env.filters['hello'] == jinja_udf  
    def test_resolve_template_files_value(self):