**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
tests/models/test_dag.py
"""
        Test if the schedule_interval will be auto aligned with the start_date
        such that if the start_date coincides with the schedule the first
        execution_date will be start_date, otherwise it will be start_date +
        interval.
        """
        dag = DAG(
            dag_id='test_scheduler_auto_align_1',
            start_date=timezone.datetime(2016, 1, 1, 10, 10, 0),
            schedule_interval="4 5 * * *",
        )
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        next_date = dag.next_dagrun_after_date(None)
        assert next_date == timezone.datetime(2016, 1, 2, 5, 4)
        dag = DAG(
            dag_id='test_scheduler_auto_align_2',
            start_date=timezone.datetime(2016, 1, 1, 10, 10, 0),
            schedule_interval="10 10 * * *",
        )
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        next_date = dag.next_dagrun_after_date(None)
        assert next_date == timezone.datetime(2016, 1, 1, 10, 10)
    def test_next_dagrun_after_not_for_subdags(self):
        """
        Test the subdags are never marked to have dagruns created, as they are
        handled by the SubDagOperator, not the scheduler
        """
        def subdag(parent_dag_name, child_dag_name, args):