**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
"""
        Calculates the previous schedule for this dag in UTC
        :param dttm: utc datetime
        :return: utc datetime
        """
        if isinstance(self.normalized_schedule_interval, str):
            
            
            dttm = pendulum.instance(dttm)
            naive = timezone.make_naive(dttm, self.timezone)
            cron = croniter(self.normalized_schedule_interval, naive)
            
            if not self.is_fixed_time_schedule():
                
                delta = naive - cron.get_prev(datetime)
                previous = dttm.in_timezone(self.timezone) - delta
            else:
                
                naive = cron.get_prev(datetime)
                tz = pendulum.timezone(self.timezone.name)
                previous = timezone.make_aware(naive, tz)
            return timezone.convert_to_utc(previous)
        elif self.normalized_schedule_interval is not None:
            return timezone.convert_to_utc(dttm - self.normalized_schedule_interval)
    def next_dagrun_info(