Fix the missing import

diff --git a/torch/backends/cudnn/__init__.py b/torch/backends/cudnn/__init__.py
index c7cd35c..f58f0a8 100644
--- a/torch/backends/cudnn/__init__.py
+++ b/torch/backends/cudnn/__init__.py
@@ -1,3 +1,4 @@
+import os
 import ctypes
 import sys
 import torch