**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
"""Returns a boolean indicating whether this DAG is paused"""
        qry = session.query(DagModel).filter(DagModel.dag_id == self.dag_id)
        return qry.value(DagModel.is_paused)
    @property
    def is_paused(self):
        """This attribute is deprecated. Please use `airflow.models.DAG.get_is_paused` method."""
        warnings.warn(
            "This attribute is deprecated. Please use `airflow.models.DAG.get_is_paused` method.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_is_paused()
    @property
    def normalized_schedule_interval(self) -> Optional[ScheduleInterval]:
        """
        Returns Normalized Schedule Interval. This is used internally by the Scheduler to
        schedule DAGs.
        1. Converts Cron Preset to a Cron Expression (e.g ``@monthly`` to ``0 0 1 * *``)
        2. If Schedule Interval is "@once" return "None"
        3. If not (1) or (2) returns schedule_interval
        """
        if isinstance(self.schedule_interval, str) and self.schedule_interval in cron_presets:
            _schedule_interval = cron_presets.get(self.schedule_interval)  
        elif self.schedule_interval == '@once':
            _schedule_interval = None
        else:
            _schedule_interval = self.schedule_interval
        return _schedule_interval
    @provide_session
    def handle_callback(self, dagrun, success=True, reason=None, session=None):