code: def _add_chars_field(data_bundle, lower=False):
      r"""
      给data_bundle中的dataset中复制一列chars. 并根据lower参数判断是否需要小写化
  
      :param data_bundle:
      :param bool lower:是否要小写化
      :return: 传入的DataBundle
      """
      data_bundle.copy_field(
          field_name="raw_chars",
          new_field_name="chars",
          ignore_miss_dataset=True,
      )
  
      if lower:
          for name, dataset in data_bundle.datasets.items():
              dataset["chars"].lower()
      return data_bundle
  
file_path: fastNLP/fastNLP/io/pipe/utils.py
imports: from typing import List
  from ...core.vocabulary import Vocabulary
  from fastNLP.core.log import logger
  from pkg_resources import parse_version
label_desc: Use image node_id 8 for calling a global function with example usage: _add_chars_field(data_bundle, lower) and returns: data_bundle