**Description**

Similarly to `@dag` and `@task` decorators, we can add a decorator to easily generate `TaskGroup` instances. 

**Use case / motivation**

This would be used for reusable pieces of DAGs. An example may be to hyperparameter tune an ML pipeline. You may want to generate several parallel subpipelines with different settings/parameters to be executed in parallel. At the end pull the result and decide the best parametrization.  

```py
@task
collect_dataset(...)
  pass

@task
def train_model(...)
   pass

@task_group
def movielens_model_pipeline(learning_rate: int, feature_list: list, dataset: XComArg):  
  dataset = filter_dataset(dataset, feature_list)
  train_model(dataset, learning_rate)

@dag
def movielens_hpt(dataset_path: str, feature_list:list=['year', 'director']):
  dataset = load_dataset(dataset_path)
  for i in range(0.1, 0.9, step=0.1):
    movielens_model_pipeline(i, feature_list, dataset)
  
  decide_best_model(...)
```




**Related Issues**

https://github.com/apache/airflow/issues?q=is%3Aopen+is%3Aissue+label%3AAIP-31


tests/utils/test_task_group.py
"""
    Tests that prefix_group_id turns on/off prefixing of task_id with group_id.
    """
    execution_date = pendulum.parse("20200101")
    with DAG("test_build_task_group_with_prefix", start_date=execution_date) as dag:
        task1 = DummyOperator(task_id="task1")
        with TaskGroup("group234", prefix_group_id=False) as group234:
            task2 = DummyOperator(task_id="task2")
            with TaskGroup("group34") as group34:
                task3 = DummyOperator(task_id="task3")
                with TaskGroup("group4", prefix_group_id=False) as group4:
                    task4 = DummyOperator(task_id="task4")
        task5 = DummyOperator(task_id="task5")
        task1 >> group234
        group34 >> task5
    assert task2.task_id == "task2"
    assert group34.group_id == "group34"
    assert task3.task_id == "group34.task3"
    assert group4.group_id == "group34.group4"
    assert task4.task_id == "task4"
    assert task5.task_id == "task5"
    assert group234.get_child_by_label("task2") == task2
    assert group234.get_child_by_label("group34") == group34
    assert group4.get_child_by_label("task4") == task4