def __getitem__(self, tokens):
raise NotImplementedError<EOL>
Looks up embedding vectors of text tokens.

        Parameters
        ----------
        tokens : str or list of strs
            A token or a list of tokens.

        Returns
        -------
        mxnet.ndarray.NDArray:
            The embedding vector(s) of the token(s). According to numpy
            conventions, if `tokens` is a string, returns a 1-D NDArray
            (vector); if `tokens` is a list of strings, returns a 2-D NDArray
            (matrix) of shape=(len(tokens), vec_len).