Correct behavior for argument gpus in Trainer (#561)

pytorch_lightning/trainer/trainer.py

@@ -132,7 +132,7 @@ class Trainer(TrainerIOMixin,
         self.gradient_clip_val = gradient_clip_val
         self.check_val_every_n_epoch = check_val_every_n_epoch
         self.track_grad_norm = track_grad_norm
-        self.on_gpu = gpus is not None and torch.cuda.is_available()
+        self.on_gpu = True if (gpus and torch.cuda.is_available()) else False
         self.process_position = process_position
         self.weights_summary = weights_summary
         self.max_nb_epochs = max_nb_epochs