**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
from airflow.exceptions import DuplicateTaskIdFound
    execution_date = pendulum.parse("20200101")
    with pytest.raises(DuplicateTaskIdFound, match=r".* 'task1' .*"):
        with DAG("test_duplicate_group_id", start_date=execution_date):
            _ = DummyOperator(task_id="task1")
            with TaskGroup("task1"):
                pass
    with pytest.raises(DuplicateTaskIdFound, match=r".* 'group1' .*"):
        with DAG("test_duplicate_group_id", start_date=execution_date):
            _ = DummyOperator(task_id="task1")
            with TaskGroup("group1", prefix_group_id=False):
                with TaskGroup("group1"):
                    pass
    with pytest.raises(DuplicateTaskIdFound, match=r".* 'group1' .*"):
        with DAG("test_duplicate_group_id", start_date=execution_date):
            with TaskGroup("group1", prefix_group_id=False):
                _ = DummyOperator(task_id="group1")
    with pytest.raises(DuplicateTaskIdFound, match=r".* 'group1.downstream_join_id' .*"):
        with DAG("test_duplicate_group_id", start_date=execution_date):
            _ = DummyOperator(task_id="task1")
            with TaskGroup("group1"):
                _ = DummyOperator(task_id="downstream_join_id")
    with pytest.raises(DuplicateTaskIdFound, match=r".* 'group1.upstream_join_id' .*"):
        with DAG("test_duplicate_group_id", start_date=execution_date):
            _ = DummyOperator(task_id="task1")
            with TaskGroup("group1"):
                _ = DummyOperator(task_id="upstream_join_id")
def test_task_without_dag():