class_docstr: 各种数据 **Loader** 的基类，提供了 API 的参考。
  :class:`Loader` 支持以下的三个函数
  
  - :meth:`download` 函数：自动将该数据集下载到缓存地址，默认缓存地址为 ``~/.fastNLP/datasets/`` 。由于版权等原因，不是所有的 ``Loader`` 都实现了该方法。
    该方法会返回下载后文件所处的缓存地址。
  - :meth:`_load` 函数：从一个数据文件中读取数据，返回一个 :class:`~fastNLP.core.DataSet` 。返回的 DataSet 的内容可以通过每个 ``Loader`` 的文档判断出。
  - :meth:`load` 函数：将文件分别读取为 :class:`~fastNLP.core.DataSet` ，然后将多个 DataSet 放入到一个 :class:`~fastNLP.io.DataBundle` 中并返回
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docstr: 自动下载该数据集
  
  :return: 下载后解压目录
docstr_tok: ["\u81ea\u52a8\u4e0b\u8f7d\u8be5\u6570\u636e\u96c6",":","return",":","\u4e0b\u8f7d\u540e\u89e3\u538b\u76ee\u5f55"]
code: def download(self) -> str:
      r"""
      自动下载该数据集
  
      :return: 下载后解压目录
      """
      raise NotImplementedError(
          f"{self.__class__} cannot download data automatically."
      )
  
file_path: fastNLP/fastNLP/io/loader/loader.py
imports: from typing import Union, Dict
  from fastNLP.io.data_bundle import DataBundle
  from fastNLP.io.file_utils import _get_dataset_url, get_cache_path, cached_path
  from fastNLP.io.utils import check_loader_paths
  from fastNLP.core.dataset import DataSet
label_desc: Use image node_id 4 for calling the Loader obj's underlying member method code with example usage: obj.download() without return types