title: TensorFlow assign concat-result tensor to tensor of different shape
body: <p>I'm using concat to append a tensor t2 of shape [150, 1] onto a tensor t1 which has shape initially of [150, 0]. I want to create a tensor which has shape [150, 1] (concatenate over axis=1), and override t1 with the new tensor. Here is my code:</p>
  
  <pre><code>t1.assign(tf.concat(1, [t1, t2]))
  </code></pre>
  
  <p>The error I'm getting is:</p>
  
  <pre><code>ValueError: Dimension 1 in both shapes must be equal, but are 0 and 1
  </code></pre>
  
  <p>Oddly enough, I'm getting the same error when I set axis=0. No idea what's going on; this should be a very easy thing to do. I suspect it has something to do with t1 having a Dimension 1 of 0. Any suggestions? Thanks in advance.</p>
  
  <p>[EDIT]</p>
  
  <p>Turns out, the problem is with "assign", as I'm setting t1 (shape (150, 0)) to the concat result (shape (150, 1)), which is making TF scream at me (rightfully so). Any way around this?</p>
  
  <p>To provide more context, t1 is a tensor which I'm using to store the logit t2 generated from the unrolling of a seq2seq RNN, in which the output sequence has an unknown length. So the size of t1 will be growing in Dimension 1 with every unrolling.</p>