stack outputs instead of concat outputs

tutorials/rnn/ptb/ptb_word_lm.py

@@ -157,7 +157,7 @@ class PTBModel(object):
         (cell_output, state) = cell(inputs[:, time_step, :], state)
         outputs.append(cell_output)
 
-    output = tf.reshape(tf.concat(axis=1, values=outputs), [-1, size])
+    output = tf.reshape(tf.stack(axis=1, values=outputs), [-1, size])
     softmax_w = tf.get_variable(
         "softmax_w", [size, vocab_size], dtype=data_type())
     softmax_b = tf.get_variable("softmax_b", [vocab_size], dtype=data_type())