**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
elif run_type and execution_date:
            if not isinstance(run_type, DagRunType):
                raise ValueError(f"`run_type` expected to be a DagRunType is {type(run_type)}")
            run_id = DagRun.generate_run_id(run_type, execution_date)
        elif not run_id:
            raise AirflowException(
                "Creating DagRun needs either `run_id` or both `run_type` and `execution_date`"
            )
        run = DagRun(
            dag_id=self.dag_id,
            run_id=run_id,
            execution_date=execution_date,
            start_date=start_date,
            external_trigger=external_trigger,
            conf=conf,
            state=state,
            run_type=run_type,
            dag_hash=dag_hash,
            creating_job_id=creating_job_id,
        )
        session.add(run)
        session.flush()
        run.dag = self
        
        
        run.verify_integrity(session=session)
        return run
    @classmethod
    @provide_session
    def bulk_sync_to_db(cls, dags: Collection["DAG"], session=None):