**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


"""
        List of leaf nodes -- ones with only upstream dependencies.
        a.k.a. the "end" of this sub-graph
        """
        raise NotImplementedError()
    @abstractmethod
    def set_upstream(self, other: DependencyMixin | Sequence[DependencyMixin]):
        """Set a task or a task list to be directly upstream from the current task."""
        raise NotImplementedError()
    @abstractmethod
    def set_downstream(self, other: DependencyMixin | Sequence[DependencyMixin]):
        """Set a task or a task list to be directly downstream from the current task."""
        raise NotImplementedError()
    def update_relative(self, other: DependencyMixin, upstream=True) -> None:
        """
        Update relationship information about another TaskMixin. Default is no-op.
        Override if necessary.
        """
    def __lshift__(self, other: DependencyMixin | Sequence[DependencyMixin]):
        """Implements Task << Task"""
        self.set_upstream(other)
        return other
    def __rshift__(self, other: DependencyMixin | Sequence[DependencyMixin]):
        """Implements Task >> Task"""
        self.set_downstream(other)
        return other
    def __rrshift__(self, other: DependencyMixin | Sequence[DependencyMixin]):