Added comment following code review.

theano/tensor/basic.py

@@ -3467,6 +3467,8 @@ class Subtensor(Op):
 
     def c_code_cache_version(self):
         hv = self.helper_c_code_cache_version()
+        # If the helper_c_code_cache_version is not versionned
+        # We don't want to have a versioned version of this op c code
         if len(hv) == 0:
             return ()
         return (1, hv)