title: Cannot convert a partially converted tensor in TensorFlow
body: <p>There are many methods in TensorFlow that requires specifying a shape, for example truncated_normal:</p>
  
  <pre><code>tf.truncated_normal(shape, mean=0.0, stddev=1.0, dtype=tf.float32, seed=None, name=None)
  </code></pre>
  
  <p>I have a placeholder for the input of shape [None, 784], where the first dimension is None because the batch size can vary. I could use a fixed batch size but it still would be different from the test/validation set size.</p>
  
  <p>I cannot feed this placeholder to tf.truncated_normal because it requires a fully specified tensor shape. What is a simple way to having tf.truncated_normal accept different tensor shapes?</p>