make sure the return result is consistent for ResultCache

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/376

Make sure the return result is consistent for ResultCache
* When gather = True, it will always return a list.

Reviewed By: itomatik

Differential Revision: D39874274

fbshipit-source-id: ce042261432cdce5544d10b2d4b88f5e2d0d1b68

d2go/evaluation/evaluator.py

@@ -80,8 +80,8 @@ class ResultCache(object):
         gather (bool): gather cache results arcoss ranks to a list
         """
         if self.cache_file is None or not PathManager.exists(self.cache_file):
-            return None
-
+            ret = None
+        else:
             with PathManager.open(self.cache_file, "rb") as fp:
                 ret = torch.load(fp)
             logger.info(f"Loaded from checkpoint {self.cache_file}")