I don’t think it is guaranteed to be connected to that same session.
On such case, I would cache the stage every couple epochs and just leave the machine on. If you get disconnected, at least you have the cache and you could continue training from there.
Also, as far as I know, you need you machine to be active. From time to time Colab asks the user to interact with the session. Though there are some javascript snippets (e.g. here 2  and here 1) to do that interaction manually, the computer would still need to be on.