**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
"""This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method."""
        warnings.warn(
            "This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_latest_execution_date()
    @property
    def subdags(self):
        """Returns a list of the subdag objects associated to this DAG"""
        
        from airflow.operators.subdag import SubDagOperator
        subdag_lst = []
        for task in self.tasks:
            if (
                isinstance(task, SubDagOperator)
                or
                
                type(task).__name__ == 'SubDagOperator'
                or task.task_type == 'SubDagOperator'
            ):
                subdag_lst.append(task.subdag)
                subdag_lst += task.subdag.subdags
        return subdag_lst
    def resolve_template_files(self):
        for t in self.tasks:
            t.resolve_template_files()
    def get_template_env(self) -> jinja2.Environment: