Move DSSIMObjective to submodule (#93)

* Move DSSIMObjective to submodule

* fix losses/dssim_test.py

keras_contrib/losses/__init__.py

@@ -0,0 +1 @@
+from .dssim import DSSIMObjective

tests/keras_contrib/losses/dssim_test.py

@@ -5,9 +5,8 @@ from keras.layers import Conv2D
 from keras.models import Sequential
 from keras.optimizers import Adam
 
-from keras_contrib import losses
+from keras.losses import sparse_categorical_crossentropy
 from keras import backend as K
-from keras_contrib import backend as KC
 from keras_contrib.losses import DSSIMObjective
 
 allobj = []
@@ -32,12 +31,12 @@ def test_objective_shapes_2d():
 def test_cce_one_hot():
     y_a = K.variable(np.random.randint(0, 7, (5, 6)))
     y_b = K.variable(np.random.random((5, 6, 7)))
-    objective_output = losses.sparse_categorical_crossentropy(y_a, y_b)
+    objective_output = sparse_categorical_crossentropy(y_a, y_b)
     assert K.eval(objective_output).shape == (5, 6)
 
     y_a = K.variable(np.random.randint(0, 7, (6,)))
     y_b = K.variable(np.random.random((6, 7)))
-    assert K.eval(losses.sparse_categorical_crossentropy(y_a, y_b)).shape == (6,)
+    assert K.eval(sparse_categorical_crossentropy(y_a, y_b)).shape == (6,)
 
 
 def test_DSSIM_channels_last():