[Cherry-pick][bug fix] fix train eval set error in static mode (#29540) (#29571)

Fix Layer train eval setting failed in static mode, more details please see #29540

python/paddle/fluid/dygraph/layers.py

@@ -132,7 +132,10 @@ class Layer(core.Layer):
                 out = mylayer(x)
 
         """
-        # global setting
+        # global setting in dygraph
+        # NOTE(chenweihang): nn.Layer also can be used in static mode,
+        # but _dygraph_tracer() can not be called in static mode
+        if in_dygraph_mode():
             framework._dygraph_tracer().train_mode()
         # Layer-level setting
         self.training = True
@@ -170,7 +173,10 @@ class Layer(core.Layer):
                 print(out)
 
         """
-        # global setting
+        # global setting in dygraph
+        # NOTE(chenweihang): nn.Layer also can be used in static mode,
+        # but _dygraph_tracer() can not be called in static mode
+        if in_dygraph_mode():
             framework._dygraph_tracer().eval_mode()
         # Layer-level setting
         self.training = False

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

@@ -3701,6 +3701,23 @@ class TestLayerParameterTrainableSet(unittest.TestCase):
             self.assertFalse(net.weight.trainable)
 
 
+class TestLayerTrainingAttribute(unittest.TestCase):
+    def test_set_train_eval_in_dynamic_mode(self):
+        with fluid.dygraph.guard():
+            net = paddle.nn.Dropout()
+            net.train()
+            self.assertTrue(net.training)
+            net.eval()
+            self.assertFalse(net.training)
+
+    def test_set_train_eval_in_static_mode(self):
+        net = paddle.nn.Dropout()
+        net.train()
+        self.assertTrue(net.training)
+        net.eval()
+        self.assertFalse(net.training)
+
+
 if __name__ == '__main__':
     paddle.enable_static()
     unittest.main()