update the calculation of num_batches_per_epoch

the number of batches per epoch also depends on the number of gpus. it should be `num_batches_per_epoch = (cifar10.NUM_EXAMPLES_PER_EPOCH_FOR_TRAIN / FLAGS.batch_size / FLAGS.num_gpus)`

tutorials/image/cifar10/cifar10_multi_gpu_train.py

@@ -147,7 +147,7 @@ def train():
 
     # Calculate the learning rate schedule.
     num_batches_per_epoch = (cifar10.NUM_EXAMPLES_PER_EPOCH_FOR_TRAIN /
-                             FLAGS.batch_size)
+                             FLAGS.batch_size / FLAGS.num_gpus)
     decay_steps = int(num_batches_per_epoch * cifar10.NUM_EPOCHS_PER_DECAY)
 
     # Decay the learning rate exponentially based on the number of steps.