code: def testGradients(self):
      t = [
          dtypes.float32,
          dtypes.float64,
          dtypes.complex64,
          dtypes.complex128,
      ]
      for src_t in t:
          for dst_t in t:
              with self.cached_session():
                  x = constant_op.constant(1.0, src_t)
  
                  def cast(x, dst_t=dst_t):
                      x = array_ops.identity(x)
                      x = math_ops.cast(x, dst_t)
                      return x
  
                  err = gradient_checker_v2.max_error(
                      *gradient_checker_v2.compute_gradient(cast, [x])
                  )
                  self.assertLess(err, 1e-3)
  
imports: import numpy
  from tensorflow.python.eager import context
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import sparse_tensor
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import gen_state_ops
  from tensorflow.python.ops import gradient_checker_v2
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import variables
  from tensorflow.python.platform import test
label_desc: Use image node_id 17 for calling the CastOpTest obj's underlying member method code with example usage: obj.testGradients() and returns: x