removed debugging assert

nnet.py

@@ -187,7 +187,6 @@ class GpuCrossentropySoftmax1HotWithBiasDx (Op):
     def make_node(self, dy, sm, y_idx):
         return Apply(self, [dy, sm, y_idx],[sm.type()])
     def perform(self, node, input_storage, output_storage):
-        assert False
         raise NotImplementedError('only C is implemented')
 
     def c_code_cache_version(self):