title: GPU cannot be indentified to run module
body: <p>I have 2 GPU machine available to use with ID 2 and 3, and would like to use them all to fit model. Here is my code, </p>
  
  <pre><code>os.environ['CUDA_VISIBLE_DEVICES'] = '2, 3'
  </code></pre>
  
  <p>with tf,device('/gpu:2'):</p>
  
  <pre><code>critic_model.fit(x,y,epochs =10)
  </code></pre>
  
  <p>with tf.device('/gpu:3'):</p>
  
  <pre><code>history = model.fit(x,y,epochs=19)
  </code></pre>
  
  <p>However, when I check nvidia-smi, I found only machine 2 is utilized, I wonder why ? </p>
  
  <p>Any idea could be helpful !</p>