https://gist.github.com/alexlee-gk/76a409f62a53883971a18a11af93241b

This was tested on a ThinkPad P70 laptop with an Intel integrated graphics and an NVIDIA GPU:

lspci | egrep 'VGA|3D'
00:02.0 VGA compatible controller: Intel Corporation Device 191b (rev 06)
01:00.0 VGA compatible controller: NVIDIA Corporation GM204GLM [Quadro M3000M] (rev a1)

A reason to use the integrated graphics for display is if installing the NVIDIA drivers causes the display to stop working properly. In my case, Ubuntu would get stuck in a login loop after installing the NVIDIA drivers. This happened regardless if I installed the drivers from the "Additional Drivers" tab in "System Settings" or the ppa:graphics-drivers/ppa in the command-line.

The solution is described in the answer from this post.

Install the NVIDIA drivers

Download the driver installation runfile from their website (e.g. the 375.26 driver runfile). Run the script with the option --no-opengl-files:

sudo ./NVIDIA-Linux-x86_64-375.26.run --no-opengl-files

Reboot.

Install CUDA

Download the CUDA installation runfile from their website. It's important to download the "runfile (local)" file so that we can explicitly prevent from overwriting the driver that was just installed. Run the script:

sudo ./cuda_8.0.44_linux.run 

Respond "no" when asked "Install NVIDIA accelerated Graphics Driver for Linux-x86_65 375.26?".

Configure xorg.conf

Modify or create the file /etx/X11/xorg.conf to specify that the NVIDIA GPU should be used for a secondary screen so that it has entries in nvidia-settings. Mine looks like this:

Section "ServerLayout"
    Identifier "layout"
    Screen 0 "intel"
    Screen 1 "nvidia"
EndSection

Section "Device"
    Identifier "intel"
    Driver "intel"
    BusID "PCI:0@0:2:0"
    Option "AccelMethod" "SNA"
EndSection

Section "Screen"
    Identifier "intel"
    Device "intel"
EndSection

Section "Device"
    Identifier "nvidia"
    Driver "nvidia"
    BusID "PCI:1@0:0:0"
    Option "ConstrainCursor" "off"
EndSection

Section "Screen"
    Identifier "nvidia"
    Device "nvidia"
    Option "AllowEmptyInitialConfiguration" "on"
    Option "IgnoreDisplayDevices" "CRT"
EndSection

The BusID should match what the lspci command above returned.

Reboot.

Miscellaneous tips

If installing the NVIDIA drivers messes up the OS (e.g. it gets stuck in a login loop or it shows a black screen before the login step), you can uninstall the drivers by openning a console with Ctrl-Alt-F1 and running:

sudo apt-get remove --purge nvidia-*
sudo apt-get autoremove
echo "nouveau" | sudo tee -a /etc/modules

Reboot.