code: def testIteratedGradientsPersistent(self):
      @polymorphic_function.function
      def _forward(z):
          return math_ops.cos(z)
  
      f = _forward
      with backprop.GradientTape(persistent=True) as tape:
          start = constant_op.constant(1.0)
          tape.watch(start)
          x = f(start)
          for expected in _COS_DERIVATIVES:
              self.assertAllClose(expected(start), x)
              x = tape.gradient(x, start)
  
imports: from absl.testing import parameterized
  from tensorflow.python.eager import backprop
  from tensorflow.python.eager import context
  from tensorflow.python.eager.polymorphic_function import polymorphic_function
  from tensorflow.python.framework import config
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import indexed_slices
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor_shape
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import cond
  from tensorflow.python.ops import gradients_impl
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import nn_grad
  from tensorflow.python.ops import nn_ops
  from tensorflow.python.ops import resource_variable_ops
  from tensorflow.python.ops import variable_scope
  from tensorflow.python.ops import variables
  from tensorflow.python.ops import while_loop
  from tensorflow.python.platform import test
label_desc: Use image node_id 8 for calling the FunctionGradientsTest obj's underlying member method code with example usage: obj.testIteratedGradientsPersistent() and returns: math_ops