use tf.gfile in saver (fix #345)

tensorpack/callbacks/saver.py

@@ -45,7 +45,7 @@ class ModelSaver(Callback):
         self.var_collections = var_collections
         if checkpoint_dir is None:
             checkpoint_dir = logger.LOG_DIR
-        assert os.path.isdir(checkpoint_dir), checkpoint_dir
+        assert tf.gfile.IsDirectory(checkpoint_dir), checkpoint_dir
         self.checkpoint_dir = checkpoint_dir
 
     def _setup_graph(self):