**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
"""
        Returns the list of dag runs between start_date (inclusive) and end_date (inclusive).
        :param start_date: The starting execution date of the DagRun to find.
        :param end_date: The ending execution date of the DagRun to find.
        :param session:
        :return: The list of DagRuns found.
        """
        dagruns = (
            session.query(DagRun)
            .filter(
                DagRun.dag_id == self.dag_id,
                DagRun.execution_date >= start_date,
                DagRun.execution_date <= end_date,
            )
            .all()
        )
        return dagruns
    @provide_session
    def get_latest_execution_date(self, session=None):
        """Returns the latest date for which at least one dag run exists"""
        return session.query(func.max(DagRun.execution_date)).filter(DagRun.dag_id == self.dag_id).scalar()
    @property
    def latest_execution_date(self):