install_gpu.sh: Enhancement [File exists check, Configure proxy if behind corporation proxy]

[nullbyte91]

I found few issue's with install_gpu.sh when I was trying to set up fast.ai using this script like,

1. We need to reboot the system after installation of cuda to execute nvidia-smi.
2. If your behind a corporation proxy we have configure conda and pip to use proxy.
3. There is no File exists check