allow optimizer fx to return 1 or 2 lists

pytorch_lightning/testing_models/lm_test_module.py

@@ -181,7 +181,9 @@ class LightningTestModel(LightningModule):
         """
         # try no scheduler for this model (testing purposes)
         optimizer = optim.Adam(self.parameters(), lr=self.hparams.learning_rate)
-        return [optimizer], []
+
+        # test returning only 1 list instead of 2
+        return [optimizer]
 
     def __dataloader(self, train):
         # init data generators