I haven’t installed CUDA before. CUDA 7.5 is not supported in my system as I have Visual Studio 2015 (on windows 10). Therefore, I am wondering if I can Install CUDA 8 toolkit instead of CUDA 7.5. I have GeForce 840M GPU. does GeForce 840M impelments the NVIDIA Pascal Architecture? Is my system compatible with CUDA 8 toolkit?
Yes, you can use CUDA 8. Your GPU is not a Pascal GPU but that is not necessary to use CUDA 8. If you want to use VS 2015, at the moment you are restricted to VS 2015 update 1 with CUDA 8RC. You may want to read the install guide, which is linked from the download page where you download CUDA 8RC.