Return a nested dict associating each variable name to its value and label.

        Returns
        -------
        dict

        Examples
        --------
        >>> df = pd.DataFrame([[1, 2], [3, 4]], columns=["col_1", "col_2"])
        >>> time_stamp = pd.Timestamp(2000, 2, 29, 14, 21)
        >>> path = "/My_path/filename.dta"
        >>> value_labels = {"col_1": {3: "x"}}
        >>> df.to_stata(path, time_stamp=time_stamp,  # doctest: +SKIP
        ...             value_labels=value_labels, version=None)  # doctest: +SKIP
        >>> with pd.io.stata.StataReader(path) as reader:  # doctest: +SKIP
        ...     print(reader.value_labels())  # doctest: +SKIP
        {'col_1': {3: 'x'}}
        >>> pd.read_stata(path)  # doctest: +SKIP
            index col_1 col_2
        0       0    1    2
        1       1    x    4
        )