Create a `LabelLists` with empty sets from the serialized file in `path/fn`.

def load_empty(cls, path:PathOrStr, fn:PathOrStr='export.pkl'):
        "Create a `LabelLists` with empty sets from the serialized file in `path/fn`."
        path = Path(path)
        state = torch.load(open(path/fn, 'rb'))
        return LabelLists.load_state(path, state)