**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
t._upstream_task_ids = t.upstream_task_ids.intersection(dag.task_dict.keys())
            t._downstream_task_ids = t.downstream_task_ids.intersection(dag.task_dict.keys())
        if len(dag.tasks) < len(self.tasks):
            dag.partial = True
        return dag
    def has_task(self, task_id: str):
        return task_id in (t.task_id for t in self.tasks)
    def get_task(self, task_id: str, include_subdags: bool = False) -> BaseOperator:
        if task_id in self.task_dict:
            return self.task_dict[task_id]
        if include_subdags:
            for dag in self.subdags:
                if task_id in dag.task_dict:
                    return dag.task_dict[task_id]
        raise TaskNotFound(f"Task {task_id} not found")
    def pickle_info(self):
        d = {}
        d['is_picklable'] = True
        try:
            dttm = timezone.utcnow()
            pickled = pickle.dumps(self)
            d['pickle_len'] = len(pickled)
            d['pickling_duration'] = str(timezone.utcnow() - dttm)
        except Exception as e:
            self.log.debug(e)
            d['is_picklable'] = False
            d['stacktrace'] = traceback.format_exc()
        return d
    @provide_session
    def pickle(self, session=None) -> DagPickle: