Returns a list of document lengths in words

        Returns
        -------
        np.array

def get_doc_lengths(self):
        '''
        Returns a list of document lengths in words

        Returns
        -------
        np.array
        '''
        idx_to_delete_list = self._build_term_index_list(True, self._get_non_unigrams())
        unigram_X, _ = self._get_X_after_delete_terms(idx_to_delete_list)
        return unigram_X.sum(axis=1).A1