code: def testDoesNotGenerateUnnecessaryGradients(self):
      operators = [
          CreateOperator("Direct", "in", "hidden"),
          CreateOperator("Direct", "hidden", "out"),
      ]
      desired_grad_operators = [
          CreateOperator("DirectGradient", "hidden_grad", "in_grad"),
      ]
      for op in desired_grad_operators:
          op.debug_info = ""
      gradients, _ = GradientRegistry.GetBackwardPass(
          operators, {"hidden": "hidden_grad"}
      )
      self.assertOperatorListEqual(gradients, desired_grad_operators)
  
imports: from hypothesis import given, settings
  import hypothesis.strategies
  import unittest
  from caffe2.proto import caffe2_pb2
  from caffe2.python import core, test_util, workspace
  from caffe2.python.core import CreateOperator, GradientRegistry, IR
  import numpy
label_desc: Use image node_id 4 for calling the TestGradientCalculation obj's underlying member method code with example usage: obj.testDoesNotGenerateUnnecessaryGradients() without return types