**Apache Airflow version**

13faa6912f7cd927737a1dc15630d3bbaf2f5d4d

**Environment**

- **Configuration**: Local Executor
- **OS** (e.g. from /etc/os-release): Mac OS 11.3
- **Kernel**: Darwin Kernel Version 20.4.0
- **Install tools**: `pip install -e .`

**The DAG**

```python
@task
def one():
    return 1

@task_group
def trivial_group(inval):

    @task
    def add_one(i):
        return i + 1

    outval = add_one(inval)
    return outval

@task
def print_it(inval):
    print(inval)

@dag(schedule_interval=None, start_date=days_ago(1), default_args={"owner": "airflow"})
def wrap():

    x = one()
    y = trivial_group(x)
    z = print_it(y)

wrap_dag = wrap()
```

**What happened**:
`print_it` had no predecessors and receives `<airflow.utils.task_group.TaskGroup object at 0x128921940>`

**What you expected to happen**:
`print_it` comes after `trivial_group.add_one` and receives `2`

The caller ends up with the task group itself, equivalent in the traditional api to `tg_ref` in:
```
with TaskGroup("trivial_group") tg_ref:
    pass
````

This interrupts the ability to continue using the Task Flow api because passing it into a function annotated with `@task` fails to register the dependency with whatever magic gets it out of xcom and adds edges to the dag.

**To Replicate**

```
 $ airflow dags test wrap $(date "+%Y-%m-%d")
```
tests/utils/test_task_group.py
"""Dummy Task3"""
        return f'[ Task3 {value} ]'
    @task
    def task_4(value):
        """Dummy Task3"""
        print(f'[ Task4 {value} ]')
    
    @task_group_decorator
    def section_1(value):
        """TaskGroup for grouping related Tasks"""
        @task_group_decorator()
        def section_2(value2):
            """TaskGroup for grouping related Tasks"""
            return task_4(task_3(value2))
        op1 = task_2(task_1(value))
        return section_2(op1)
    execution_date = pendulum.parse("20201109")
    with DAG(
        dag_id="example_nested_task_group_decorator", start_date=execution_date, tags=["example"]
    ) as dag:
        t_start = task_start()
        sec_1 = section_1(t_start)
        sec_1.set_downstream(task_end())
    
    assert set(dag.task_group.children.keys()) == {"task_start", "task_end", "section_1"}
    assert set(dag.task_group.children['section_1'].children.keys()) == {
        'section_1.task_1',
        'section_1.task_2',
        'section_1.section_2',