title: unknown size from stacked tensorArray in tf.while_loop output
body: <p>The following code uses a <code>tf.while_loop(...)</code> for computations of a dynamic length. </p>
  
  <pre><code>    outputs_tensor_array = tf.TensorArray(tf.float32,
                                            size=0,
                                            clear_after_read=False,
                                            infer_shape=False,
                                            dynamic_size = True,
                                            element_shape[self.batch_size, self.size])
  
      initial_args = [outputs_tensor_array, 0]
      outputs, *_ = tf.while_loop(lambda out, idx, *_ : idx &lt; max_len,
                                  func,
                                  initial_args + additional_args,
                                  parallel_iterations = 32,
                                  swap_memory = True)
      outputs = outputs.stack()
  </code></pre>
  
  <p>I'm wondering if its possible to enforce a size, or atleast make that size be <code>None</code> in order to enforce a size constraint and enable further computations down the graph. The current shape is <code>[?, batch, hidden_size]</code></p>