**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
tis = session.query(TI)
        dag_ids = []
        if include_subdags:
            
            conditions = []
            for dag in self.subdags + [self]:
                conditions.append((TI.dag_id == dag.dag_id) & TI.task_id.in_(dag.task_ids))
                dag_ids.append(dag.dag_id)
            tis = tis.filter(or_(*conditions))
        else:
            tis = session.query(TI).filter(TI.dag_id == self.dag_id)
            tis = tis.filter(TI.task_id.in_(self.task_ids))
        if include_parentdag and self.is_subdag and self.parent_dag is not None:
            p_dag = self.parent_dag.sub_dag(
                task_ids_or_regex=r"^{}$".format(self.dag_id.split('.')[1]),
                include_upstream=False,
                include_downstream=True,
            )
            tis = tis.union(
                p_dag.clear(
                    start_date=start_date,
                    end_date=end_date,
                    only_failed=only_failed,
                    only_running=only_running,
                    confirm_prompt=confirm_prompt,
                    include_subdags=include_subdags,
                    include_parentdag=False,
                    dag_run_state=dag_run_state,
                    get_tis=True,
                    session=session,