**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
"""Test that @dag uses function name as default dag id."""
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def noop_pipeline():
            @task_decorator
            def return_num(num):
                return num
            return_num(4)
        dag = noop_pipeline()
        assert isinstance(dag, DAG)
        assert dag.dag_id, 'noop_pipeline'
    def test_documentation_added(self):
        """Test that @dag uses function docs as doc_md for DAG object"""
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def noop_pipeline():
            """
            Regular DAG documentation
            """
            @task_decorator
            def return_num(num):
                return num
            return_num(4)
        dag = noop_pipeline()
        assert isinstance(dag, DAG)
        assert dag.dag_id, 'test'
        assert dag.doc_md.strip(), "Regular DAG documentation"
    def test_fails_if_arg_not_set(self):
        """Test that @dag decorated function fails if positional argument is not set"""
        @dag_decorator(default_args=self.DEFAULT_ARGS)
        def noop_pipeline(value):