allow to do a_var.transfer(another_var.type.context_name) on both back-end.

theano/sandbox/cuda/type.py

@@ -42,6 +42,8 @@ class CudaNdarrayType(Type):
 
     dtype = 'float32'
 
+    context_name = 'gpu'  # For similar interface with the new back-end.
+
     Variable = None
     """
     This will be set to the Variable type corresponding to this class.