**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
"""Verify if dag.leaves returns the leaf tasks of a DAG."""
        with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = DummyOperator(task_id="t1")
            op2 = DummyOperator(task_id="t2")
            op3 = DummyOperator(task_id="t3")
            op4 = DummyOperator(task_id="t4")
            op5 = DummyOperator(task_id="t5")
            [op1, op2] >> op3 >> [op4, op5]
            assert set(dag.leaves) == {op4, op5}
    def test_tree_view(self):
        """Verify correctness of dag.tree_view()."""
        with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = DummyOperator(task_id="t1")
            op2 = DummyOperator(task_id="t2")
            op3 = DummyOperator(task_id="t3")
            op1 >> op2 >> op3
            with redirect_stdout(io.StringIO()) as stdout:
                dag.tree_view()
                stdout = stdout.getvalue()
            stdout_lines = stdout.split("\n")
            assert 't1' in stdout_lines[0]
            assert 't2' in stdout_lines[1]
            assert 't3' in stdout_lines[2]
    def test_duplicate_task_ids_not_allowed_with_dag_context_manager(self):