'Abstract method that returns an iterable of TIDepStatus objects that describe
whether the given task instance has this dependency met.
For example a subclass could return an iterable of TIDepStatus objects, each one
representing if each of the passed in task\'s upstream tasks succeeded or not.
:param ti: the task instance to get the dependency status for
:type ti: TaskInstance
:param session: database session
:type session: Session
:param dep_context: the context for which this dependency should be evaluated for
:type dep_context: DepContext'

def _get_dep_statuses(self, ti, session, dep_context=None):

    raise NotImplementedError
