cmake: fix CMAKE_CUDA_ARCHITECTURES default (#2421)

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-backend/CMakeLists.txt

@@ -65,6 +65,10 @@ if (LLMODEL_VULKAN)
     list(APPEND BUILD_VARIANTS vulkan vulkan-avxonly)
 endif()
 if (LLMODEL_CUDA)
+    if (DEFINED CMAKE_CUDA_ARCHITECTURES)
+        set(GGML_CUDA_ARCHITECTURES "${CMAKE_CUDA_ARCHITECTURES}")
+    endif()
+
     include(CheckLanguage)
     check_language(CUDA)
     if (NOT CMAKE_CUDA_COMPILER)