**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
is_paused_upon_creation: Optional[bool] = None,
        jinja_environment_kwargs: Optional[Dict] = None,
        tags: Optional[List[str]] = None,
    ):
        from airflow.utils.task_group import TaskGroup
        self.user_defined_macros = user_defined_macros
        self.user_defined_filters = user_defined_filters
        self.default_args = copy.deepcopy(default_args or {})
        self.params = params or {}
        
        if 'params' in self.default_args:
            self.params.update(self.default_args['params'])
            del self.default_args['params']
        validate_key(dag_id)
        self._dag_id = dag_id
        self._full_filepath = full_filepath if full_filepath else ''
        self._concurrency = concurrency
        self._pickle_id: Optional[int] = None
        self._description = description
        
        back = sys._getframe().f_back
        self.fileloc = back.f_code.co_filename if back else ""
        self.task_dict: Dict[str, BaseOperator] = {}
        
        if start_date and start_date.tzinfo:
            self.timezone = start_date.tzinfo
        elif 'start_date' in self.default_args and self.default_args['start_date']:
            if isinstance(self.default_args['start_date'], str):
                self.default_args['start_date'] = timezone.parse(self.default_args['start_date'])
            self.timezone = self.default_args['start_date'].tzinfo