fix compile

paddle/operators/multiplex_op.cu

@@ -71,7 +71,7 @@ class MultiplexGradGPUKernel : public framework::OpKernel<T> {
     index_t_cpu.CopyFrom(*ids, platform::CPUPlace(), ctx.device_context());
     auto* index = index_t_cpu.data<int32_t>();
 
-    auto stream = ctx.device_context().stream();
+    auto stream = ctx.cuda_device_context().stream();
     Place place = boost::get<Place>(ctx.GetPlace());
     for (auto i = 0; i < rows; i++) {
       size_t k = static_cast<size_t>(index[i]);