added arg docs

pytorch_lightning/models/trainer.py

@@ -30,9 +30,14 @@ except ModuleNotFoundError:
 
 def reduce_distributed_output(output, nb_gpus):
     pdb.set_trace()
-    if nb_gpus <= 1 or type(output) is torch.Tensor:
+    if nb_gpus <= 1:
         return output
 
+    # when using DP, we get one output per gpu
+    # average outputs and return
+    if type(output) is torch.Tensor:
+        return output.mean()
+
     for k, v in output.items():
         # recurse on nested dics
         if isinstance(output[k], dict):