Rename files to .cu and fix IDX2C usage

src/utils/cuda_helper.cu

@@ -0,0 +1,28 @@
+#include <cstdio>
+#include <cstdlib>
+#include "cuda_helper.h"
+#include <cuda_runtime.h>
+#include <cublas_v2.h>
+
+cudaDeviceProp initializeCUDA(cublasHandle_t& cublasHandle) {
+    int deviceCount;
+    CUDA_CHECK(cudaGetDeviceCount(&deviceCount));
+
+    if (deviceCount == 0) {
+        std::fprintf(stderr, "No CUDA devices found. Exiting.\n");
+        std::exit(EXIT_FAILURE);
+    }
+
+    int device = 0;
+    CUDA_CHECK(cudaSetDevice(device));
+
+    cudaDeviceProp deviceProp;
+    CUDA_CHECK(cudaGetDeviceProperties(&deviceProp, device));
+
+    std::printf("Using CUDA device %d: %s\n", device, deviceProp.name);
+
+    // Initialize cuBLAS
+    CUBLAS_CHECK(cublasCreate(&cublasHandle));
+
+    return deviceProp;
+}