Fix sparse_categorical_crossentropy argument and add value test for it (

#7513) (#7524)

* Add value test for sparse_categorical_crossentropy (#7513)

* Swap sparse_categorical_crossentropy's target for output

keras/backend/cntk_backend.py

@@ -1629,7 +1629,7 @@ def categorical_crossentropy(target, output, from_logits=False):
 def sparse_categorical_crossentropy(target, output, from_logits=False):
     target = C.one_hot(target, output.shape[-1])
     target = C.reshape(target, output.shape)
-    return categorical_crossentropy(output, target, from_logits)
+    return categorical_crossentropy(target, output, from_logits)
 
 
 class Function(object):

tests/keras/losses_test.py

@@ -57,5 +57,14 @@ def test_categorical_hinge():
     assert np.isclose(expected_loss, np.mean(loss))
 
 
+def test_sparse_categorical_crossentropy():
+    y_pred = K.variable(np.array([[0.3, 0.6, 0.1],
+                                  [0.1, 0.2, 0.7]]))
+    y_true = K.variable(np.array([1, 2]))
+    expected_loss = - (np.log(0.6) + np.log(0.7)) / 2
+    loss = K.eval(losses.sparse_categorical_crossentropy(y_true, y_pred))
+    assert np.isclose(expected_loss, np.mean(loss))
+
+
 if __name__ == '__main__':
     pytest.main([__file__])