class_docstr: 执行评测时使用的 backend，是所有 backend 的父类。Backend 及其子类的所有方法都必须是无状态的。
class_docstr_tok: ["\u6267\u884c\u8bc4\u6d4b\u65f6\u4f7f\u7528\u7684","backend\uff0c\u662f\u6240\u6709","backend","\u7684\u7236\u7c7b\u3002Backend","\u53ca\u5176\u5b50\u7c7b\u7684\u6240\u6709\u65b9\u6cd5\u90fd\u5fc5\u987b\u662f\u65e0\u72b6\u6001\u7684\u3002"]
code: def __init__(self):
      self._specified = False
  
file_path: fastNLP/fastNLP/core/metrics/backend/backend.py
imports: from ..utils import AggregateMethodError
label_desc: Use image node_id 1 to create a new Backend object  with example: obj = Backend()