Doc about GpuOp.

doc/cifarSC2011/extending_theano.txt

@@ -211,5 +211,11 @@ Exercises 8
 
   - Our current element-wise fusion generates computation with only 1 output.
 
-
-
+GPU Op
+------
+
+Op that execute on the GPU should inherit from the
+``theano.sandbox.cuda.GpuOp`` and not ``theano.Op``. This allow Theano
+to make the distinction between both. Currently, we use this to test
+if the NVIDIA driver work correctly with our sum reduction code on the
+gpu.