def _split_into_words(self, sentences):
        """
        Splits multiple sentences into words and flattens the result

        Args:
          sentences: list of string

        Returns:
          A list of words (split by white space)
        """
        # Modified from https://github.com/pltrdy/seq2seq/blob/master/seq2seq/metrics/rouge.py
        if self.word_split is None:
            return list(itertools.chain(*[_.split() for _ in sentences]))
        else:
            return list(itertools.chain(*[self.word_split(_) for _ in sentences]))

        Splits multiple sentences into words and flattens the result

        Args:
          sentences: list of string

        Returns:
          A list of words (split by white space)
        
utils_nlp/eval/rouge/rouge_ext.py