Update tf.concat_v2 to tf.concat

tutorials/image/cifar10/cifar10_multi_gpu_train.py

@@ -124,7 +124,7 @@ def average_gradients(tower_grads):
       grads.append(expanded_g)
 
     # Average over the 'tower' dimension.
-    grad = tf.concat_v2(grads, 0)
+    grad = tf.concat(grads, 0)
     grad = tf.reduce_mean(grad, 0)
 
     # Keep in mind that the Variables are redundant because they are shared