make gpu_context inside macro

paddle/pybind/pybind.cc

@@ -136,18 +136,14 @@ All parameter, weight, gradient are variables in Paddle.
                   []() -> paddle::platform::DeviceContext* {
                     return new paddle::platform::CPUDeviceContext();
                   })
+#ifndef PADDLE_ONLY_CPU
       .def_static("gpu_context",
                   [](paddle::platform::Place& place)
                       -> paddle::platform::DeviceContext* {
-#ifdef PADDLE_ONLY_CPU
-
-                    // PADDLE_THROW("'GPUPlace' is not supported in CPU only
-                    // device.");
-                    return nullptr;
-#else
                     return new paddle::platform::CUDADeviceContext(place);
+                  })
 #endif
-                  });
+      ;
 
   py::class_<paddle::platform::Place>(m, "GPUPlace").def(py::init<int>());