Fix dygraph train mode

test=develop

python/paddle/fluid/dygraph/layers.py

@@ -49,10 +49,10 @@ class Layer(core.Layer):
         self._helper = LayerObjectHelper(self._full_name)
 
     def train(self):
-        framework._dygraph_tracer()._train_mode()
+        framework._dygraph_tracer().train_mode()
 
     def eval(self):
-        framework._dygraph_tracer()._eval_mode()
+        framework._dygraph_tracer().eval_mode()
 
     def full_name(self):
         """Full name for this layers.
@@ -261,10 +261,10 @@ class PyLayer(core.PyLayer):
         super(PyLayer, self).__init__()
 
     def train(self):
-        framework._dygraph_tracer()._train_mode()
+        framework._dygraph_tracer().train_mode()
 
     def eval(self):
-        framework._dygraph_tracer()._eval_mode()
+        framework._dygraph_tracer().eval_mode()
 
     @classmethod
     def _do_forward(cls, inputs):

python/paddle/fluid/dygraph/tracer.py

@@ -118,8 +118,8 @@ class Tracer(core.Tracer):
                     if k in backward_refs:
                         op.backward_refs[k] = outputs[k]
 
-    def _train_mode(self):
+    def train_mode(self):
         self._train_mode = True
 
-    def _eval_mode(self):
+    def eval_mode(self):
         self._train_mode = False

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

@@ -117,6 +117,7 @@ class TestImperativeMnist(unittest.TestCase):
             train_reader = paddle.batch(
                 paddle.dataset.mnist.train(), batch_size=128, drop_last=True)
 
+            mnist.train()
             dy_param_init_value = {}
             for epoch in range(epoch_num):
                 for batch_id, data in enumerate(train_reader()):