class_docstr: 所有 **Padder** 对象的父类，所有的 Padder 对象都会实现静态函数 ``pad(batch_field, pad_val=0, dtype=None)`` 。
code: def pad(batch_field, pad_val=0, dtype=None):
      raise NotImplementedError()
  
label_desc: Use image node_id 3 for calling the Padder obj's underlying member method code with example usage: obj.pad(batch_field, pad_val, dtype) without return types