Fix error when running on cpu with MODEL.DEVICE="cpu"

Summary: The assert just below fails because `backend = "NCCL"` and we don't have a GPU

Reviewed By: ppwwyyxx

Differential Revision: D31506095

fbshipit-source-id: c1199eeb732d098c02fe5cd40efb85284deaa3b9

d2go/distributed.py

@@ -72,7 +72,7 @@ def launch(
             )
             with temp_defrost(cfg):
                 cfg.MODEL.DEVICE = "cpu"
-            backend = "GLOO"
+        backend = "GLOO"
 
     if backend == "NCCL":
         assert (