**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
self,
        dag_id: str,
        description: Optional[str] = None,
        schedule_interval: Optional[ScheduleInterval] = timedelta(days=1),
        start_date: Optional[datetime] = None,
        end_date: Optional[datetime] = None,
        full_filepath: Optional[str] = None,
        template_searchpath: Optional[Union[str, Iterable[str]]] = None,
        template_undefined: Type[jinja2.StrictUndefined] = jinja2.StrictUndefined,
        user_defined_macros: Optional[Dict] = None,
        user_defined_filters: Optional[Dict] = None,
        default_args: Optional[Dict] = None,
        concurrency: int = conf.getint('core', 'dag_concurrency'),
        max_active_runs: int = conf.getint('core', 'max_active_runs_per_dag'),
        dagrun_timeout: Optional[timedelta] = None,
        sla_miss_callback: Optional[Callable] = None,
        default_view: str = conf.get('webserver', 'dag_default_view').lower(),
        orientation: str = conf.get('webserver', 'dag_orientation'),
        catchup: bool = conf.getboolean('scheduler', 'catchup_by_default'),
        on_success_callback: Optional[DagStateChangeCallback] = None,
        on_failure_callback: Optional[DagStateChangeCallback] = None,
        doc_md: Optional[str] = None,
        params: Optional[Dict] = None,
        access_control: Optional[Dict] = None,