Add a name

There is an assertion about name, so I add this.

Author: YinongLong

example/tutorial_mnist_simple.py

@@ -33,7 +33,7 @@
 
 # define cost function and metric.
 y = network.outputs
-cost = tl.cost.cross_entropy(y, y_)
+cost = tl.cost.cross_entropy(y, y_, name='xentropy')
 correct_prediction = tf.equal(tf.argmax(y, 1), y_)
 acc = tf.reduce_mean(tf.cast(correct_prediction, tf.float32))
 y_op = tf.argmax(tf.nn.softmax(y), 1)