**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
execution_date = pendulum.parse("20200101")
    with DAG("test_build_task_group_context_manager", start_date=execution_date) as dag:
        task1 = DummyOperator(task_id="task1")
        with TaskGroup("group234") as group234:
            _ = DummyOperator(task_id="task2")
            with TaskGroup("group34") as group34:
                _ = DummyOperator(task_id="task3")
                _ = DummyOperator(task_id="task4")
        task5 = DummyOperator(task_id="task5")
        task1 >> group234
        group34 >> task5
    assert task1.get_direct_relative_ids(upstream=False) == {
        'group234.group34.task4',
        'group234.group34.task3',
        'group234.task2',
    }
    assert task5.get_direct_relative_ids(upstream=True) == {
        'group234.group34.task4',
        'group234.group34.task3',
    }
    assert dag.task_group.group_id is None
    assert dag.task_group.is_root
    assert set(dag.task_group.children.keys()) == {"task1", "group234", "task5"}
    assert group34.group_id == "group234.group34"
    assert task_group_to_dict(dag.task_group) == EXPECTED_JSON
def test_build_task_group():