class_docstr: 提供给用户使用的 ``state``，用来记载您的 ``callback`` 实时数据，该 ``state`` 实际上是一个字典，我们通过复用 ``__getattr__`` 方法来实现类似
  类属性的字典调用方式；
  
  为了实现断点重训，用户应当保证其保存的信息都是可序列化的；
  
  推荐的使用方式::
  
      >>> state = State()
      >>> state["best_accuracy"] = 0.9
      >>> print(state["best_accuracy"])
      or
      >>> print(state.best_accuracy)
code: def __getattr__(self, item):
      if item in self:
          _value = self[item]
          if isinstance(_value, dict):
              return State(_value)
          else:
              return _value
      else:
          raise ValueError(f"key '{item}' is not existed!")
  
file_path: fastNLP/fastNLP/core/controllers/utils/state.py
imports: from dataclasses import dataclass
  from typing import Optional, Dict
label_desc: Use image node_id 2 for calling the State obj's underlying member method code with example usage: obj.__getattr__(item) and returns: State, _value