Reduced some C++ compiler warnings (#6197)

* Removed some warnings

* Rebase with master

* Solved C++ Google Tests errors made by refactoring in order to remove warnings

* Undo renaming path -> path_

* Fix style check

Co-authored-by: Hyunsu Cho <chohyu01@cs.washington.edu>

rabit/src/c_api.cc

@@ -26,10 +26,10 @@ struct FHelper {
 template<typename DType>
 struct FHelper<op::BitOR, DType> {
   static void
-  Allreduce(DType *senrecvbuf_,
-            size_t count,
-            void (*prepare_fun)(void *arg),
-            void *prepare_arg) {
+  Allreduce(DType *,
+            size_t ,
+            void (*)(void *arg),
+            void *) {
     utils::Error("DataType does not support bitwise or operation");
   }
 };
@@ -196,7 +196,7 @@ struct ReadWrapper : public Serializable {
                     "Read pickle string");
     }
   }
-  void Save(Stream *fo) const override {
+  void Save(Stream *) const override {
     utils::Error("not implemented");
   }
 };
@@ -208,7 +208,7 @@ struct WriteWrapper : public Serializable {
                         size_t length)
       : data(data), length(length) {
   }
-  void Load(Stream *fi) override {
+  void Load(Stream *) override {
     utils::Error("not implemented");
   }
   void Save(Stream *fo) const override {

rabit/src/engine.cc

@@ -96,8 +96,8 @@ void Allreduce_(void *sendrecvbuf,  // NOLINT
                 size_t type_nbytes,
                 size_t count,
                 IEngine::ReduceFunction red,
-                mpi::DataType dtype,
-                mpi::OpType op,
+                mpi::DataType,
+                mpi::OpType ,
                 IEngine::PreprocFunction prepare_fun,
                 void *prepare_arg) {
   GetEngine()->Allreduce(sendrecvbuf, type_nbytes, count, red, prepare_fun,
@@ -112,7 +112,7 @@ int ReduceHandle::TypeSize(const MPI::Datatype &dtype) {
   return static_cast<int>(dtype.type_size);
 }
 
-void ReduceHandle::Init(IEngine::ReduceFunction redfunc, size_t type_nbytes) {
+void ReduceHandle::Init(IEngine::ReduceFunction redfunc, size_t ) {
   utils::Assert(redfunc_ == nullptr, "cannot initialize reduce handle twice");
   redfunc_ = redfunc;
 }