**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
new_start = self.previous_schedule(last_start)
            if self.start_date:
                if new_start >= self.start_date:
                    self.start_date = new_start
            else:
                self.start_date = new_start
        next_run_date = None
        if not date_last_automated_dagrun:
            
            task_start_dates = [t.start_date for t in self.tasks if t.start_date]
            if task_start_dates:
                next_run_date = self.normalize_schedule(min(task_start_dates))
                self.log.debug("Next run date based on tasks %s", next_run_date)
        else:
            next_run_date = self.following_schedule(date_last_automated_dagrun)
        if date_last_automated_dagrun and next_run_date:
            while next_run_date <= date_last_automated_dagrun:
                next_run_date = self.following_schedule(next_run_date)
        
        if self.start_date:
            next_run_date = self.start_date if not next_run_date else max(next_run_date, self.start_date)
            if next_run_date == self.start_date:
                next_run_date = self.normalize_schedule(self.start_date)
            self.log.debug("Dag start date: %s. Next run date: %s", self.start_date, next_run_date)
        
        if next_run_date and self.end_date and next_run_date > self.end_date:
            return None
        
        
        task_end_dates = [t.end_date for t in self.tasks if t.end_date]