class_docstr: Feature First Difference
  Parameters
  ----------
  feature : Expression
      feature instance
  Returns
  ----------
  Expression
      a feature instance with first difference
code: def get_extended_window_size(self):
      lft_etd, rght_etd = self.feature.get_extended_window_size()
      return lft_etd + 1, rght_etd
  
file_path: qlib/tests/test_register_ops.py
imports: import unittest
  import numpy
  from qlib.data import D
  from qlib.data.ops import ElemOperator, PairOperator
  from qlib.tests import TestAutoData
label_desc: Use image node_id 2 for calling the Diff obj's underlying member method code with example usage: obj.get_extended_window_size() and returns: unknown,  rght_etd