**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
include_downstream=True,
                            )
                            tis = tis.union(
                                downstream.clear(
                                    start_date=tii.execution_date,
                                    end_date=tii.execution_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,
                                    recursion_depth=recursion_depth + 1,
                                    max_recursion_depth=max_recursion_depth,
                                    dag_bag=dag_bag,
                                    visited_external_tis=visited_external_tis,
                                )
                            )
                        visited_external_tis.add(ti_key)
        if get_tis:
            return tis
        tis = tis.all()
        if dry_run:
            session.expunge_all()
            return tis
        
        
        count = len(tis)