Dense matrix needs RAJA when GPU is used (#676)

* dense matrix needs RAJA when GPU is used

* update CMakeList

* add a description

* have an error message when GPU is on but raja is off.

CMakeLists.txt

@@ -222,6 +222,10 @@ else()
 endif()
 
 if(HIOP_USE_GPU)
+  if(NOT HIOP_USE_RAJA)
+    message(FATAL_ERROR "Error: HIOP_USE_GPU is ON, but HIOP_USE_RAJA is OFF. HiOp requires using RAJA with CUDA/HIP.")
+  endif()
+
   include(CheckLanguage)
   if(HIOP_USE_MAGMA)
     set(HIOP_MAGMA_DIR CACHE PATH "Path to Magma directory")

src/LinAlg/LinAlgFactory.cpp

@@ -353,7 +353,7 @@ hiopMatrixDense* LinearAlgebraFactory::create_matrix_dense(const ExecSpaceInfo&
       return nullptr;
 #endif //HIOP_USE_RAJA
     } else {// for if(hi.exec_backend_ == "RAJA")
-
+      assert(false && "device memory backend not available because HiOp was not built with RAJA");
       if(mem_space_upper == "CUDA") {
 #ifdef HIOP_USE_CUDA
           assert(mem_space_upper == "DEVICE");