SA: for #958: removing root gpu hack in trainer/evaluation_loop

pytorch_lightning/trainer/evaluation_loop.py

@@ -429,9 +429,13 @@ def evaluation_forward(self, model, batch, batch_idx, dataloader_idx, test_mode:
         # single GPU data transfer
         if self.single_gpu:
             # for single GPU put inputs on gpu manually
-            root_gpu = 0
+
             if isinstance(self.data_parallel_device_ids, list):
                 root_gpu = self.data_parallel_device_ids[0]
+            else:
+                raise RuntimeError(
+                    'Expected `data_parallel_device_ids` as a list, cannot determine root gpu.'
+                )
             batch = self.transfer_batch_to_gpu(batch, root_gpu)
             args[0] = batch