**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,
        date_last_automated_dagrun: Optional[pendulum.DateTime],
    ) -> Tuple[Optional[pendulum.DateTime], Optional[pendulum.DateTime]]:
        """
        Get information about the next DagRun of this dag after ``date_last_automated_dagrun`` -- the
        execution date, and the earliest it could be scheduled
        :param date_last_automated_dagrun: The max(execution_date) of existing
            "automated" DagRuns for this dag (scheduled or backfill, but not
            manual)
        """
        if (
            self.schedule_interval == "@once" and date_last_automated_dagrun
        ) or self.schedule_interval is None:
            
            return (None, None)
        next_execution_date = self.next_dagrun_after_date(date_last_automated_dagrun)
        if next_execution_date is None:
            return (None, None)
        if self.schedule_interval == "@once":
            
            return (next_execution_date, next_execution_date)
        return (next_execution_date, self.following_schedule(next_execution_date))
    def next_dagrun_after_date(self, date_last_automated_dagrun: Optional[pendulum.DateTime]):