This is only when using the deprecated interface: if no data_specs is provided, but "topo" is True, then the data is returned in the Conv2DSpace that was implicitly provided when building the data set, which is stored in `self.X_topo_space`.
If the data set was built from a matrix, without any view_converter, and we create an iterator by calling `self.iterator(topo=True)`, then there is no way of knowing how to convert it.