**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
return "task_2"
    @task
    def task_3():
        return "task_3"
    @task
    def task_4(task_2_output, task_3_output):
        print(task_2_output, task_3_output)
    @task
    def task_5():
        print("task_5")
    execution_date = pendulum.parse("20200101")
    with DAG("test_build_task_group_with_task_decorator", start_date=execution_date) as dag:
        tsk_1 = task_1()
        with TaskGroup("group234") as group234:
            tsk_2 = task_2()
            tsk_3 = task_3()
            tsk_4 = task_4(tsk_2, tsk_3)
        tsk_5 = task_5()
        tsk_1 >> group234 >> tsk_5
    
    assert tsk_1.operator in tsk_2.operator.upstream_list
    assert tsk_1.operator in tsk_3.operator.upstream_list
    assert tsk_5.operator in tsk_4.operator.downstream_list
    
    assert extract_node_id(task_group_to_dict(dag.task_group)) == {
        'id': None,
        'children': [
            {
                'id': 'group234',
                'children': [