class_docstr: Feature First Difference
  Parameters
  ----------
  feature : Expression
      feature instance
  Returns
  ----------
  Expression
      a feature instance with first difference
code: def _load_internal(self, instrument, start_index, end_index, freq):
      series = self.feature.load(
          instrument, start_index, end_index, freq
      )
      return series.diff()
  
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 1 for calling the Diff obj's underlying member method code with example usage: obj._load_internal(instrument, start_index, end_index, freq) and returns: series