bulk_create doesn't work with multi-table inheritance

[cfytrok]

Replace bulk_create with iteration in atomic transaction in a multi-table inheritance scenario.

if is_multitabel_inheritance(self.model):
      with transaction.atomic():
          for obj in objs:
               obj.save()
          return objs

def is_multitabel_inheritance(cls):
    for parent in cls._meta.get_parent_list():
        if parent._meta.concrete_model is not cls._meta.concrete_model:
            return True
    return False