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 __call__(self, batch_field):
      return self.pad(
          batch_field=batch_field,
          pad_val=self.pad_val,
          dtype=self.dtype,
      )
  
label_desc: Use image node_id 2 for calling the Padder obj's underlying member method code with example usage: obj.__call__(batch_field) and returns: self