title: What do the options in ConfigProto like allow_soft_placement and log_device_placement mean?
body: <p>We see this quite often in many of the TensorFlow tutorials:</p>
  
  <pre class="lang-py prettyprint-override"><code>sess = tf.Session(config=tf.ConfigProto(allow_soft_placement=True, 
                                          log_device_placement=True))
  </code></pre>
  
  <p>What does <code>allow_soft_placement</code> and <code>log_device_placement</code> mean?</p>