cuda: fix compile error for cuda backend

If we compile cuda backend, we must set different
cuda nvcc path to CMAKE_CUDA_COMPILER for diff
operating system, it's too troublesome.

Signed-off-by: lizhenneng <lizhenneng@kylinos.cn>

ggml/src/ggml-cuda/CMakeLists.txt

@@ -38,6 +38,13 @@ if (CUDAToolkit_FOUND)
     endif()
     message(STATUS "Using CUDA architectures: ${CMAKE_CUDA_ARCHITECTURES}")
 
+    if(NOT DEFINED CMAKE_CUDA_COMPILER)
+        if(WIN32)
+            set(CMAKE_CUDA_COMPILER "${CUDAToolkit_BIN_DIR}/nvcc.exe")
+        else()
+            set(CMAKE_CUDA_COMPILER "${CUDAToolkit_BIN_DIR}/nvcc")
+        endif()
+    endif()
     enable_language(CUDA)
 
     file(GLOB   GGML_HEADERS_CUDA "*.cuh")