langchain/text_splitter.py/NLTKTextSplitter/__init__
class NLTKTextSplitter:    def __init__(self, separator: str = "\n\n", **kwargs: Any):
        """Initialize the NLTK splitter."""
        super().__init__(**kwargs)
        try:
            from nltk.tokenize import sent_tokenize

            self._tokenizer = sent_tokenize
        except ImportError:
            raise ImportError(
                "NLTK is not installed, please install it with `pip install nltk`."
            )
        self._separator = separator