Create a `LabelLists` with empty sets from the serialized `state`.

def load_state(cls, path:PathOrStr, state:dict):
        "Create a `LabelLists` with empty sets from the serialized `state`."
        path = Path(path)
        train_ds = LabelList.load_state(path, state)
        valid_ds = LabelList.load_state(path, state)
        return LabelLists(path, train=train_ds, valid=valid_ds)