**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
DEFAULT_ARGS = {
        "owner": "test",
        "depends_on_past": True,
        "start_date": timezone.utcnow(),
        "retries": 1,
        "retry_delay": timedelta(minutes=1),
    }
    DEFAULT_DATE = timezone.datetime(2016, 1, 1)
    VALUE = 42
    def setUp(self):
        super().setUp()
        self.operator = None
    def tearDown(self):
        super().tearDown()
        clear_db_runs()
    def test_set_dag_id(self):
        """Test that checks you can set dag_id from decorator."""
        @dag_decorator('test', default_args=self.DEFAULT_ARGS)
        def noop_pipeline():
            @task_decorator
            def return_num(num):
                return num
            return_num(4)
        dag = noop_pipeline()
        assert isinstance(dag, DAG)
        assert dag.dag_id, 'test'
    def test_default_dag_id(self):