fix a bug in testing only returning loss_style

Author: Taited

tests/test_losses/test_gen_auxiliary_loss.py

@@ -109,7 +109,7 @@ def test_only_style_loss(self):
             data_info=self.data_info, perceptual_weight=0)
         loss_style = perceptual_loss(dict(fake_imgs=pred, real_imgs=target))
         assert loss_style.shape == ()
-        assert perceptual_loss.perceptual_weight is None
+        assert perceptual_loss.perceptual_weight == 0
 
     def test_with_different_layer_weights(self):
         unknown_h, unknown_w = (32, 32)