**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,
        start_date: datetime,
        num: Optional[int] = None,
        end_date: Optional[datetime] = timezone.utcnow(),
    ) -> List[datetime]:
        if num is not None:
            end_date = None
        return utils_date_range(
            start_date=start_date, end_date=end_date, num=num, delta=self.normalized_schedule_interval
        )
    def is_fixed_time_schedule(self):
        """
        Figures out if the DAG schedule has a fixed time (e.g. 3 AM).
        :return: True if the schedule has a fixed time, False if not.
        """
        now = datetime.now()
        cron = croniter(self.normalized_schedule_interval, now)
        start = cron.get_next(datetime)
        cron_next = cron.get_next(datetime)
        if cron_next.minute == start.minute and cron_next.hour == start.hour:
            return True
        return False
    def following_schedule(self, dttm):