**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
recursion_depth=recursion_depth,
                    max_recursion_depth=max_recursion_depth,
                    dag_bag=dag_bag,
                    visited_external_tis=visited_external_tis,
                )
            )
        if start_date:
            tis = tis.filter(TI.execution_date >= start_date)
        if end_date:
            tis = tis.filter(TI.execution_date <= end_date)
        if only_failed:
            tis = tis.filter(or_(TI.state == State.FAILED, TI.state == State.UPSTREAM_FAILED))
        if only_running:
            tis = tis.filter(TI.state == State.RUNNING)
        if task_ids:
            tis = tis.filter(TI.task_id.in_(task_ids))
        if include_subdags:
            from airflow.sensors.external_task import ExternalTaskMarker
            
            instances = tis.all()
            for ti in instances:
                if ti.operator == ExternalTaskMarker.__name__:
                    if visited_external_tis is None:
                        visited_external_tis = set()
                    ti_key = ti.key.primary
                    if ti_key not in visited_external_tis:
                        
                        
                        task: ExternalTaskMarker = cast(
                            ExternalTaskMarker, copy.copy(self.get_task(ti.task_id))