code: def contribute_to_class(self, cls, name):
          
          super(EmbeddedMediaField, self).contribute_to_class(cls, name)
          register_field(cls, self)
          
          # add a virtual field that will create signals on any/all subclasses
          cls._meta.add_virtual_field(EmbeddedSignalCreator(self))
docs: I need a way to ensure that this signal gets created for all child
          models, and since model inheritance doesn't have a 'contrubite_to_class'
          style hook, I am creating a fake virtual field which will be added to
          all subclasses and handles creating the signal