I read in the UBI Forums that you will be able to play in OFFLINE mode if you want and it was Ubisoft Support saying so answering a query. You have to activate the game online first.
I don't want to keep wondering about using a trainer so I opened a support ticket and asked them directly if I can use one in offline mode or not. Should have an answer within 48 hours or so.
Link
[Edited by element5, 11/16/2016 2:11:11 AM]
They will probably give you the same generic info that pretty much every game says. Don't tamper with or use any tool made to alter the game, etc etc. We won't know anything for sure until we try it.
The game launched on consoles yesterday and the online modes didn't even work, yet the review teams were able to complete the campaign mode with no problem.