**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


airflow/utils/task_group.py
"""Required by TaskMixin"""
        return list(self.get_leaves())
    def get_roots(self) -> Generator["BaseOperator", None, None]:
        """
        Returns a generator of tasks that are root tasks, i.e. those with no upstream
        dependencies within the TaskGroup.
        """
        for task in self:
            if not any(self.has_task(parent) for parent in task.get_direct_relatives(upstream=True)):
                yield task
    def get_leaves(self) -> Generator["BaseOperator", None, None]:
        """
        Returns a generator of tasks that are leaf tasks, i.e. those with no downstream
        dependencies within the TaskGroup
        """
        for task in self:
            if not any(self.has_task(child) for child in task.get_direct_relatives(upstream=False)):
                yield task
    def child_id(self, label):
        """
        Prefix label with group_id if prefix_group_id is True. Otherwise return the label
        as-is.
        """
        if self.prefix_group_id and self.group_id:
            return f"{self.group_id}.{label}"
        return label
    @property
    def upstream_join_id(self) -> str: