**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
f_kwargs[name] = dag_obj.param(name, value)
                
                f(**f_kwargs)
            
            return dag_obj
        return factory
    return wrapper
STATICA_HACK = True
globals()['kcah_acitats'[::-1].upper()] = False
if STATICA_HACK:  
    from airflow.models.serialized_dag import SerializedDagModel
    DagModel.serialized_dag = relationship(SerializedDagModel)
class DagContext:
    """
    DAG context is used to keep the current DAG when DAG is used as ContextManager.
    You can use DAG as context:
    .. code-block:: python
        with DAG(
            dag_id='example_dag',
            default_args=default_args,
            schedule_interval='0 0 * * *',
            dagrun_timeout=timedelta(minutes=60)
        ) as dag:
    If you do this the context stores the DAG and whenever new task is created, it will use
    such stored DAG as the parent DAG.
    """
    _context_managed_dag: Optional[DAG] = None
    _previous_context_managed_dags: List[DAG] = []
    @classmethod
    def push_context_managed_dag(cls, dag: DAG):