Allow depending to a @task_group as a whole

[uranusjr]

Fix #19903.

Instead of always using the task returned by the task group function as the dependency representation, also allow a task group function to not return any task specifically.

Setting dependency to such a group would depend on the group's contained tasks as a whole, and not any specific task in it.

This renders this DAG definition taken from #19903 (comment) (which would fail to parse currently)

@dag(start_date=datetime(2023, 1, 1), schedule_interval="@once")
def test_dag_1():
    @task
    def start():
        pass

    @task
    def do_thing(x):
        print(x)

    @task_group
    def do_all_things():
        do_thing(1)
        do_thing(2)

    @task
    def end():
        pass

    start() >> do_all_things() >> end()

my_dag = test_dag_1()

like this:

Tests to come later. Done

[josh-fell]

Love that this gets to parity with the context manager version of TaskGroups!

[eladkal]

Nice!

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[josh-fell]

Doesn't need to be in this PR but it might be nice to have some documentation on these two behaviors with the @task_group decorator.

@uranusjr I can log a request for this if you don't want to take this up. Let me know.

[josh-fell]

Actually looks like this would be all new documentation on the @task_group decorator. I can't find any docs in 2.2.3 that outline the feature (at least not easily) and no docs seem to reference the [START/END howto_task_group_decorator] markers in the example_task_group_decorator DAG too.

[uranusjr]

I created #20762 to track documentation changes.