**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
)
                        ti.task = task
                        if recursion_depth == 0:
                            
                            
                            max_recursion_depth = task.recursion_depth
                        if recursion_depth + 1 > max_recursion_depth:
                            
                            raise AirflowException(
                                "Maximum recursion depth {} reached for {} {}. "
                                "Attempted to clear too many tasks "
                                "or there may be a cyclic dependency.".format(
                                    max_recursion_depth, ExternalTaskMarker.__name__, ti.task_id
                                )
                            )
                        ti.render_templates()
                        external_tis = session.query(TI).filter(
                            TI.dag_id == task.external_dag_id,
                            TI.task_id == task.external_task_id,
                            TI.execution_date == pendulum.parse(task.execution_date),
                        )
                        for tii in external_tis:
                            if not dag_bag:
                                dag_bag = DagBag(read_dags_from_db=True)
                            external_dag = dag_bag.get_dag(tii.dag_id)
                            if not external_dag:
                                raise AirflowException(f"Could not find dag {tii.dag_id}")
                            downstream = external_dag.sub_dag(
                                task_ids_or_regex=fr"^{tii.task_id}$",
                                include_upstream=False,