title: Why cannot the value be fed into the `placeholder_with_default' in TensorFlow?
body: <p>I just used TensorFlow (version 1.8.0) to train a model. Here, I used <code>tf.placeholder_with_default</code> to initialize a placeholder:</p>
  
  <p><code>t_iteration = tf.placeholder_with_default(-1, [], name='t_iteration')</code> (so the default value of <code>t_iteration</code> is -1 if there is no value fed into the placeholder. After that, I used <code>sess.run(t_iteration, feed_dict={t_iteration: 2})</code> to try to feed a value of 2 into this placeholder. </p>
  
  <p>However, it is very strange that when I used <code>B = t_iteration.eval(session=sess)</code> to convert the tensor to an array and check the value that I have fed, the B always equals to -1 not 2. I don't know the reason about this? Seems the <code>sess.run(t_iteration, feed_dict={t_iteration: 2})</code> didn't work.</p>
  
  <p>Could you help me with this issue?</p>
  
  <p>Thanks! </p>