feat(wandb): display logger
Logger info when `wandb` not installed was set to `info` which does not display by default.
It has been changed to `warning`.
It don't think it should really be a warning if you don't use wandb.

The root issue here is probably discoverability of wandb for users who don't know it? Then it would probably be better solved in documentation.

We will start some documentation on `Trainer`/`TFTrainer` in the coming weeks (cc @LysandreJik) we'll mention wandb there. (if you want to help with this let us know)<|||||>Makes sense, let me know when it's started and I can help writing the section related to wandb.