Thanks Jeremy. Thanks to your answer, I found in “Known issues” on the GPC website the following warning that confirms both my problem and your tmux solution:
**Intermittent disconnects** : At this time, we do not offer a specific SLA for connection lifetimes. Use terminal multiplexers like [tmux](https://tmux.github.io/) or [screen](http://www.gnu.org/software/screen/) if you plan to keep the terminal window open for an extended period of time.
I hope this will also help other fastai users to train DL models online on huge datasets like LMs from scratch.
[ EDIT ] For people without technical background : the idea is to launch your jupyter notebook from a session opened online (on GCP for example) through tmux and not from your ubuntu terminal in your computer. Thus, even if your ssh connection stops, the session used to launch your jupyter notebook is still running online 
More information in this post.