[Refactor] set USE_LLVM to optional. (#476)

Author: hyx1999

docs/get_started/Installation.md

@@ -133,7 +133,7 @@ Copy the configuration file and enable the desired backends (e.g., LLVM and CUDA
 mkdir build
 cp 3rdparty/tvm/cmake/config.cmake build
 cd build
-echo "set(USE_LLVM ON)" >> config.cmake
+# echo "set(USE_LLVM ON)"  # set USE_LLVM to ON if using LLVM
 echo "set(USE_CUDA ON)" >> config.cmake 
 # or echo "set(USE_ROCM ON)" >> config.cmake to enable ROCm runtime
 cmake ..