cuda: prevent CMAKE_CUDA_ARCHITECTURES default from overiding OpenCV architecture search

CMakeLists.txt

@@ -726,7 +726,13 @@ if(ENABLE_CUDA_FIRST_CLASS_LANGUAGE)
 
     cmake_policy(SET CMP0092 NEW) # CMake 3.15+: leave warning flags out of default CMAKE_<LANG>_FLAGS flags.
     if(CMAKE_CUDA_COMPILER)
+      if(CMAKE_CUDA_ARCHITECTURES)
+        set(USER_DEFINED_CMAKE_CUDA_ARCHITECTURES TRUE)
+      endif()
       enable_language(CUDA)
+      if(NOT USER_DEFINED_CMAKE_CUDA_ARCHITECTURES)
+        set(CMAKE_CUDA_ARCHITECTURES "")
+      endif()
     elseif(UNIX)
       message(WARNING "CUDA: Not detected!  If you are not using the default host compiler (g++) then you need to specify both CMAKE_CUDA_HOST_COMPILER and CMAKE_CUDA_COMPILER. e.g. -DCMAKE_CUDA_HOST_COMPILER=/usr/bin/clang++ -DCMAKE_CUDA_COMPILER=/usr/local/cuda/bin/nvcc.")
     endif()