Fix import issue in training.py.

The issue was I used a module which wasn't imported in 2.2. The import line was only added after the 2.2 branch cut.

tensorflow/python/keras/engine/training_test.py

@@ -1367,7 +1367,7 @@ class TrainingTest(keras_parameterized.TestCase):
   @keras_parameterized.run_all_keras_modes(always_skip_v1=True)
   def test_gradients_are_none(self):
 
-    class DenseWithExtraWeight(layers_module.Dense):
+    class DenseWithExtraWeight(keras.layers.Dense):
 
       def build(self, input_shape):
         # Gradients w.r.t. extra_weights are None
@@ -1377,9 +1377,9 @@ class TrainingTest(keras_parameterized.TestCase):
         self.extra_weight_2 = self.add_weight('extra_weight_2', shape=(),
                                               initializer='ones')
 
-    model = sequential.Sequential([DenseWithExtraWeight(4, input_shape=(4,))])
+    model = keras.models.Sequential([DenseWithExtraWeight(4, input_shape=(4,))])
     # Test clipping can handle None gradients
-    opt = optimizer_v2.adam.Adam(clipnorm=1.0, clipvalue=1.0)
+    opt = keras.optimizer_v2.adam.Adam(clipnorm=1.0, clipvalue=1.0)
     model.compile(opt, 'mse', run_eagerly=testing_utils.should_run_eagerly())
     inputs = np.random.normal(size=(64, 4))
     targets = np.random.normal(size=(64, 4))