title: Keras multi_gpu_model returns error 'tensorflow_core._api.v2.config' has no attribute 'experimental_list_devices'
body: <p>I have problems training my unet model on both my gpu's, </p>
  
  <p>The model is a simple U-net implementation that i know works, since its testet without being a multi_gpu_model</p>
  
  <pre><code>train_generator = zip(image_generator, mask_generator)
  
  with tf.device("/cpu:0"):
      # initialize the model
      model = unet((512,512,3))
  
  # make the model parallel
  model = multi_gpu_model(model, gpus=2)
  model.compile(optimizer='adam', loss="mean_squared_error")
  
  model.fit_generator(train_generator, steps_per_epoch=250, epochs=10)
  
  :output
  File "C:/Users/PycharmProjects/U-net/U-net.py", line 29, in &lt;module&gt;
      model = multi_gpu_model(model, gpus=2)
    File "C:\Users\AppData\Local\Programs\Python\Python37\lib\site-packages\keras\utils\multi_gpu_utils.py", line 150, in multi_gpu_model
      available_devices = _get_available_devices()
    File "C:\Users\AppData\Local\Programs\Python\Python37\lib\site-packages\keras\utils\multi_gpu_utils.py", line 16, in _get_available_devices
      return K.tensorflow_backend._get_available_gpus() + ['/cpu:0']
    File "C:\Users\AppData\Local\Programs\Python\Python37\lib\site-packages\keras\backend\tensorflow_backend.py", line 506, in _get_available_gpus
      _LOCAL_DEVICES = tf.config.experimental_list_devices()
  AttributeError: module 'tensorflow_core._api.v2.config' has no attribute 'experimental_list_devices'
  </code></pre>
  
  <p>I have also tried tf.distributed.mirroredStrategy() but also without luck</p>
  
  <p>any help would be greatly appreciated </p>