One of the biggest challenge (aside from upgrading nvidia & cuda) in upgrading from Ubuntu 16.04 to Ubuntu 18.04 is enabling GPU passthrough in LXD Containers. Due to a bug in parsing logic, it cannot properly parse Blacklisted line in nvidia-410 drivers.
Only solution is to upgrade lxd to 3.0.2. To do that you have to enable pre-released updates (bionic-proposed).
Then just upgrade lxd:
sudo apt upgrade lxd
Reboot and you are done.