**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
"""Build a Jinja2 environment."""
        
        searchpath = [self.folder]
        if self.template_searchpath:
            searchpath += self.template_searchpath
        
        jinja_env_options = {
            'loader': jinja2.FileSystemLoader(searchpath),
            'undefined': self.template_undefined,
            'extensions': ["jinja2.ext.do"],
            'cache_size': 0,
        }
        if self.jinja_environment_kwargs:
            jinja_env_options.update(self.jinja_environment_kwargs)
        env = jinja2.Environment(**jinja_env_options)  
        
        
        if self.user_defined_macros:
            env.globals.update(self.user_defined_macros)
        if self.user_defined_filters:
            env.filters.update(self.user_defined_filters)
        return env
    def set_dependency(self, upstream_task_id, downstream_task_id):
        """
        Simple utility method to set dependency between two tasks that
        already have been added to the DAG using add_task()
        """
        self.get_task(upstream_task_id).set_downstream(self.get_task(downstream_task_id))
    @provide_session
    def get_task_instances(self, start_date=None, end_date=None, state=None, session=None):