**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
assert extract_node_id(task_group_to_dict(dag.task_group), include_label=True) == {
        'id': None,
        'label': None,
        'children': [
            {
                'id': 'group234',
                'label': 'group234',
                'children': [
                    {
                        'id': 'group34',
                        'label': 'group34',
                        'children': [
                            {
                                'id': 'group34.group4',
                                'label': 'group4',
                                'children': [{'id': 'task4', 'label': 'task4'}],
                            },
                            {'id': 'group34.task3', 'label': 'task3'},
                            {'id': 'group34.downstream_join_id', 'label': ''},
                        ],
                    },
                    {'id': 'task2', 'label': 'task2'},
                    {'id': 'group234.upstream_join_id', 'label': ''},
                ],
            },
            {'id': 'task1', 'label': 'task1'},
            {'id': 'task5', 'label': 'task5'},
        ],
    }
def test_build_task_group_with_task_decorator():