**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
return get_last_dagrun(
            self.dag_id, session=session, include_externally_triggered=include_externally_triggered
        )
    @provide_session
    def has_dag_runs(self, session=None, include_externally_triggered=True) -> bool:
        return (
            get_last_dagrun(
                self.dag_id, session=session, include_externally_triggered=include_externally_triggered
            )
            is not None
        )
    @property
    def dag_id(self) -> str:
        return self._dag_id
    @dag_id.setter
    def dag_id(self, value: str) -> None:
        self._dag_id = value
    @property
    def full_filepath(self) -> str:
        return self._full_filepath
    @full_filepath.setter
    def full_filepath(self, value) -> None:
        self._full_filepath = value
    @property
    def concurrency(self) -> int:
        return self._concurrency
    @concurrency.setter
    def concurrency(self, value: int):