**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
"""
        Test `default_view` default value of DAG initialization
        """
        dag = models.DAG(dag_id='test-default_default_view')
        assert conf.get('webserver', 'dag_default_view').lower() == dag.default_view
    def test_dag_invalid_orientation(self):
        """
        Test invalid `orientation` of DAG initialization
        """
        with pytest.raises(AirflowException, match='Invalid values of dag.orientation: only support'):
            models.DAG(dag_id='test-invalid-orientation', orientation='airflow')
    def test_dag_orientation_default_value(self):
        """
        Test `orientation` default value of DAG initialization
        """
        dag = models.DAG(dag_id='test-default_orientation')
        assert conf.get('webserver', 'dag_orientation') == dag.orientation
    def test_dag_as_context_manager(self):
        """
        Test DAG as a context manager.
        When used as a context manager, Operators are automatically added to
        the DAG (unless they specify a different DAG)
        """
        dag = DAG('dag', start_date=DEFAULT_DATE, default_args={'owner': 'owner1'})
        dag2 = DAG('dag2', start_date=DEFAULT_DATE, default_args={'owner': 'owner2'})
        with dag:
            op1 = DummyOperator(task_id='op1')
            op2 = DummyOperator(task_id='op2', dag=dag2)