Assert that the C code is cpu-only rather than fallback to python.

theano/sandbox/rng_mrg.py

@@ -369,8 +369,10 @@ class mrg_uniform(mrg_uniform_base):
 
     def c_code(self, node, name, inp, out, sub):
         rstate, size = inp
-        if not isinstance(node.inputs[0].type, TensorType):
-            raise NotImplementedError("C code is cpu-only")
+        # If we try to use the C code here with something else than a
+        # TensorType, something is wrong (likely one of the GPU ops
+        # not defining C code correctly).
+        assert isinstance(node.inputs[0].type, TensorType):
         o_rstate, o_sample = out
         if self.inplace:
             o_rstate_requirement = 'NPY_ARRAY_C_CONTIGUOUS|NPY_ARRAY_ALIGNED'