**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
@task_decorator
            def return_num(num):
                return num
            assert isinstance(value, DagParam)
            xcom_arg = return_num(value)
            self.operator = xcom_arg.operator  
        dag = xcom_pass_to_op()
        new_value = 52
        dr = dag.create_dagrun(
            run_id=DagRunType.MANUAL.value,
            start_date=timezone.utcnow(),
            execution_date=self.DEFAULT_DATE,
            state=State.RUNNING,
            conf={'value': new_value},
        )
        self.operator.run(start_date=self.DEFAULT_DATE, end_date=self.DEFAULT_DATE)
        ti = dr.get_task_instances()[0]
        assert ti.xcom_pull(), new_value
    def test_set_params_for_dag(self):
        """Test that dag param is correctly set when using dag decorator"""
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def xcom_pass_to_op(value=self.VALUE):
            @task_decorator
            def return_num(num):
                return num
            xcom_arg = return_num(value)
            self.operator = xcom_arg.operator  
        dag = xcom_pass_to_op()
        assert dag.params['value'] == self.VALUE