code: def test_concrete_function_conversion(backend):
      # testing : https://coremltools.readme.io/docs/tensorflow-2#conversion-from-user-defined-models
  
      @tf.function(
          input_signature=[tf.TensorSpec(shape=(6,), dtype=tf.float32)]
      )
      def gelu_tanh_activation(x):
          a = np.sqrt(2 / np.pi) * (x + 0.044715 * tf.pow(x, 3))
          y = 0.5 * (1.0 + tf.tanh(a))
          return x * y
  
      conc_func = gelu_tanh_activation.get_concrete_function()
      mlmodel = ct.convert([conc_func], convert_to=backend[0])
  
imports: import itertools
  import os
  import platform
  from os import chdir, getcwd
  from shutil import rmtree
  from tempfile import mkdtemp
  import numpy
  import pytest
  import coremltools
  from coremltools.converters.mil.frontend.tensorflow.test.test_tf_conversion_api import TestInputOutputConversionAPI
  from coremltools.converters.mil.frontend.tensorflow.test.test_tf_conversion_api import TestiOS16DefaultIODtype
  from coremltools.converters.mil.mil import types
  from coremltools.converters.mil.testing_reqs import backends
  import tensorflow
  from tensorflow import keras
  from tensorflow.keras import layers
label_desc: Use image node_id 8 for calling the TestTensorFlow2ConverterExamples obj's underlying member method code with example usage: obj.test_concrete_function_conversion(backend) and returns: unknown