Add cross_entropy loss to mnist ut

python/paddle/fluid/tests/unittests/test_imperative_optimizer.py

@@ -125,8 +125,8 @@ class TestImperativeMnist(unittest.TestCase):
                 label._stop_gradient = True
 
                 cost = mnist(img)
-                #  loss = fluid.layers.cross_entropy(cost)
-                avg_loss = fluid.layers.reduce_mean(cost)
+                loss = fluid.layers.cross_entropy(cost, label)
+                avg_loss = fluid.layers.mean(loss)
                 dy_out = avg_loss._numpy()
 
                 if batch_id == 0:
@@ -156,8 +156,8 @@ class TestImperativeMnist(unittest.TestCase):
                 name='pixel', shape=[1, 28, 28], dtype='float32')
             label = fluid.layers.data(name='label', shape=[1], dtype='int64')
             cost = mnist(img)
-            # loss = fluid.layers.cross_entropy(cost)
-            avg_loss = fluid.layers.reduce_mean(cost)
+            loss = fluid.layers.cross_entropy(cost, label)
+            avg_loss = fluid.layers.mean(loss)
             sgd.minimize(avg_loss)
 
             # initialize params and fetch them