class_docstr: test the private functions used in post processing.
docstr: Tests the _apply_postprocessing_stack helper function.
docstr_tok: ["Tests","the","_apply_postprocessing_stack","helper","function","."]
code: def test_postprocessing_stack(self):
      """Tests the _apply_postprocessing_stack helper function."""
  
      results = (1.0, 2.0, 3.0, 4.0)
  
      def postprocessing1(results):
          return (results[0] + results[1], results[2] + results[3])
  
      def postprocessing2(results):
          return (results[0] + 1, results[1] + 2)
  
      out1 = _apply_postprocessing_stack(results, [postprocessing1])
      assert out1 == (3.0, 7.0)
  
      out2 = _apply_postprocessing_stack(
          results, [postprocessing2, postprocessing1]
      )
      assert out2 == (4.0, 9.0)
  
imports: from typing import Sequence, Callable
  import pytest
  import pennylane
  from pennylane.tape import QuantumScript
  from pennylane.transforms.core import transform, TransformProgram, TransformError, TransformContainer
  from pennylane.transforms.core.transform_program import _batch_postprocessing, _apply_postprocessing_stack, null_postprocessing
  from pennylane.typing import ResultBatch, Result
label_desc: Use image node_id 2 for calling the TestUtilityHelpers obj's underlying member method code with example usage: obj.test_postprocessing_stack() and returns: unknown,  unknown, unknown,  unknown