title: No result of tf.Print in keras's model.fit
body: <p>I wrote that loss (for testing custom loss in keras):</p>
  
  <pre><code>def loss(y_true, y_pred):
    loss = -tf.reduce_sum(y_true * tf.log(y_pred))
    loss = tf.Print(loss, [loss], 'loss = ')
  return loss
  </code></pre>
  
  <p>and then:</p>
  
  <pre><code>model.compile(loss=loss, 
                optimizer=keras.optimizers.Adadelta(),
                metrics=['accuracy'])
  model.fit(x_train, y_train)
  </code></pre>
  
  <p>and there's no tf.Print results:</p>
  
  <pre><code>Epoch 1/12 
  60000/60000 [==============================] - 12s 198us/step - loss: 25.3197 - acc: 0.9384 - val_loss: 5.6927 - val_acc: 0.9857
  Epoch 2/12
  60000/60000 [==============================] - 11s 187us/step - loss: 8.7803 - acc: 0.9798 - val_loss: 4.6938 - val_acc: 0.9888
  </code></pre>
  
  <p>Why?</p>