More refactoring to take advantage of collective aggregators (#9081)

src/collective/aggregator.h

@@ -31,18 +31,16 @@ namespace collective {
  * @param buffer The buffer storing the results.
  * @param size The size of the buffer.
  * @param function The function used to calculate the results.
- * @param args Arguments to the function.
  */
-template <typename Function, typename T, typename... Args>
-void ApplyWithLabels(MetaInfo const& info, T* buffer, size_t size, Function&& function,
-                     Args&&... args) {
+template <typename Function>
+void ApplyWithLabels(MetaInfo const& info, void* buffer, size_t size, Function&& function) {
   if (info.IsVerticalFederated()) {
     // We assume labels are only available on worker 0, so the calculation is done there and result
     // broadcast to other workers.
     std::string message;
     if (collective::GetRank() == 0) {
       try {
-        std::forward<Function>(function)(std::forward<Args>(args)...);
+        std::forward<Function>(function)();
       } catch (dmlc::Error& e) {
         message = e.what();
       }
@@ -55,7 +53,7 @@ void ApplyWithLabels(MetaInfo const& info, T* buffer, size_t size, Function&& fu
       LOG(FATAL) << &message[0];
     }
   } else {
-    std::forward<Function>(function)(std::forward<Args>(args)...);
+    std::forward<Function>(function)();
   }
 }