avoid using core function from backend

Author: yhmtsai

core/CMakeLists.txt

@@ -37,7 +37,6 @@ target_sources(
         base/dense_cache.cpp
         base/device_matrix_data.cpp
         base/executor.cpp
-        base/event.cpp
         base/index_set.cpp
         base/memory.cpp
         base/mpi.cpp

core/base/event.cpp

@@ -8,24 +8,25 @@
 #include <memory>
 
 #include <ginkgo/core/base/event.hpp>
+#include <ginkgo/core/base/executor.hpp>
 
 
 namespace gko {
 
 
-class Executor;
-
-
 /**
  * NotAsyncEvent is to provide an Event implementation on unsupported executor
  * like reference. It will ensure the kernels are finished when recording this
  * event.
  */
 class NotAsyncEvent : public Event {
 public:
-    NotAsyncEvent(std::shared_ptr<const Executor> exec);
+    NotAsyncEvent(std::shared_ptr<const Executor> exec) { exec->synchronize(); }
 
-    void synchronize() const override;
+    void synchronize() const override
+    {
+        // we have sync in the recording phase
+    }
 };