title: GPU is not used for calculations despite tensorflow-gpu installed
body: <p>My computer has the following software installed: Anaconda (3), TensorFlow (GPU), and Keras.
  There are two Anaconda virtual environments - one with TensorFlow for Python 2.7 and one for 3.5, both GPU version, <a href="https://www.tensorflow.org/install/install_linux" rel="nofollow noreferrer">installed according to the TF instructions</a>. (I had a CPU version of TensorFlow installed previously in a separate environment, but I've deleted it.)</p>
  
  <p>When I run the following:</p>
  
  <pre><code>source activate tensorflow-gpu-3.5
  python code.py
  </code></pre>
  
  <p>and check <code>nvidia-smi</code> it shows only 3MiB GPU Memory Usage by Python, so it looks like GPU is not used for calculations.
  (<code>code.py</code> is a simple deep Q-learning algorithm implemented with Keras)</p>
  
  <p>Any ideas what can be going wrong? </p>