**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
self,
        task_ids=None,
        start_date=None,
        end_date=None,
        only_failed=False,
        only_running=False,
        confirm_prompt=False,
        include_subdags=True,
        include_parentdag=True,
        dag_run_state: str = State.RUNNING,
        dry_run=False,
        session=None,
        get_tis=False,
        recursion_depth=0,
        max_recursion_depth=None,
        dag_bag=None,
        visited_external_tis=None,
    ):
        """
        Clears a set of task instances associated with the current dag for
        a specified date range.
        :param task_ids: List of task ids to clear
        :type task_ids: List[str]
        :param start_date: The minimum execution_date to clear
        :type start_date: datetime.datetime or None
        :param end_date: The maximum execution_date to clear
        :type end_date: datetime.datetime or None
        :param only_failed: Only clear failed tasks