fix ExponentialMovingAverage documentation so that ExponentialMovingAverage.apply is evaluated within control_dependencies (#12987)

tensorflow/python/training/moving_averages.py

@@ -278,14 +278,12 @@ class ExponentialMovingAverage(object):
   # Create an ExponentialMovingAverage object
   ema = tf.train.ExponentialMovingAverage(decay=0.9999)
 
-  # Create the shadow variables, and add ops to maintain moving averages
-  # of var0 and var1.
-  maintain_averages_op = ema.apply([var0, var1])
-
-  # Create an op that will update the moving averages after each training
-  # step.  This is what we will use in place of the usual training op.
   with tf.control_dependencies([opt_op]):
-      training_op = tf.group(maintain_averages_op)
+      # Create the shadow variables, and add ops to maintain moving averages
+      # of var0 and var1. This also creates an op that will update the moving
+      # averages after each training step.  This is what we will use in place
+      # of the usual training op.
+      training_op = ema.apply([var0, var1])
 
   ...train the model by running training_op...
   ```