**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
with DAG('dag', start_date=DEFAULT_DATE, default_args={'owner': 'owner1'}):
            with pytest.raises(AirflowException):
                DummyOperator(task_id='should_fail', weight_rule='no rule')
    def test_get_num_task_instances(self):
        test_dag_id = 'test_get_num_task_instances_dag'
        test_task_id = 'task_1'
        test_dag = DAG(dag_id=test_dag_id, start_date=DEFAULT_DATE)
        test_task = DummyOperator(task_id=test_task_id, dag=test_dag)
        ti1 = TI(task=test_task, execution_date=DEFAULT_DATE)
        ti1.state = None
        ti2 = TI(task=test_task, execution_date=DEFAULT_DATE + datetime.timedelta(days=1))
        ti2.state = State.RUNNING
        ti3 = TI(task=test_task, execution_date=DEFAULT_DATE + datetime.timedelta(days=2))
        ti3.state = State.QUEUED