### Body

Currently `start_date` is mandatory parameter.
For DAGs with `schedule=None` we can relax this requirement as no scheduling calculation needed so the `start_date` parameter isn't used.

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
return BaseOperator(task_id=task_id).as_teardown()
        def teardown_task_f(task_id):
            return BaseOperator(task_id=task_id).as_teardown(on_failure_fail_dagrun=True)
        def work_task(task_id):
            return BaseOperator(task_id=task_id)
        def setup_task(task_id):
            return BaseOperator(task_id=task_id).as_setup()
        def make_task(task_id):
            """
            Task factory helper.
            Will give a setup, teardown, work, or teardown-with-dagrun-failure task depending on input.
            """
            if task_id.startswith("s"):
                factory = setup_task
            elif task_id.startswith("w"):
                factory = work_task
            elif task_id.startswith("tf"):
                factory = teardown_task_f
            elif task_id.startswith("t"):
                factory = teardown_task
            else:
                raise ValueError("unexpected")
            return dag.task_dict.get(task_id) or factory(task_id=task_id)
        return (make_task(x) for x in input_str.split(", "))
    @staticmethod
    def cleared_downstream(task):