**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
airflow/models/dag.py
f'Invalid values of dag.default_view: only support '
                f'{DEFAULT_VIEW_PRESETS}, but get {default_view}'
            )
        if orientation in ORIENTATION_PRESETS:
            self.orientation = orientation
        else:
            raise AirflowException(
                f'Invalid values of dag.orientation: only support '
                f'{ORIENTATION_PRESETS}, but get {orientation}'
            )
        self.catchup = catchup
        self.is_subdag = False  
        self.partial = False
        self.on_success_callback = on_success_callback
        self.on_failure_callback = on_failure_callback
        
        
        
        self.edge_info: Dict[str, Dict[str, EdgeInfoType]] = {}
        
        
        self.has_on_success_callback = self.on_success_callback is not None
        self.has_on_failure_callback = self.on_failure_callback is not None
        self.doc_md = doc_md
        self._access_control = DAG._upgrade_outdated_dag_access_control(access_control)
        self.is_paused_upon_creation = is_paused_upon_creation
        self.jinja_environment_kwargs = jinja_environment_kwargs
        self.tags = tags
        self._task_group = TaskGroup.create_root(self)
    def __repr__(self):