### 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.
"""Set dependency between two tasks that already have been added to the DAG using add_task()."""
        self.get_task(upstream_task_id).set_downstream(self.get_task(downstream_task_id))
    @provide_session
    def get_task_instances_before(