**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
scheduler_lock = Column(Boolean)
    
    pickle_id = Column(Integer)
    
    
    
    
    fileloc = Column(String(2000))
    
    owners = Column(String(2000))
    
    description = Column(Text)
    
    default_view = Column(String(25))
    
    schedule_interval = Column(Interval)
    
    tags = relationship('DagTag', cascade='all,delete-orphan', backref=backref('dag'))
    concurrency = Column(Integer, nullable=False)
    has_task_concurrency_limits = Column(Boolean, nullable=False)
    
    next_dagrun = Column(UtcDateTime)
    
    next_dagrun_create_after = Column(UtcDateTime)
    __table_args__ = (
        Index('idx_root_dag_id', root_dag_id, unique=False),
        Index('idx_next_dagrun_create_after', next_dagrun_create_after, unique=False),
    )
    NUM_DAGS_PER_DAGRUN_QUERY = conf.getint('scheduler', 'max_dagruns_to_create_per_loop', fallback=10)
    def __init__(self, **kwargs):