[RFC] Let Logger base class inherit from Callback
🚀 Feature
We could consider having loggers inherit the Callback base class.
Motivation
This would enable third party loggers to perform additional features during the progression of training.
One example is uploading checkpoint files in WandB: #6231
It would make it significantly easier to add these features without tightly coupling them to the rest of the Trainer logic.
Also recently there was a feature request to add state persistence to loggers #6361. We would face similar challenges as we have with callbacks. But if we allow loggers to implement the callback methods, and when we solve #6467, then this would also enable #6361 to move forward without problem.
Pitch
# now
class LightningLoggerBase(ABC):

# pitch
class LightningLoggerBase(Callback, ABC):
Alternatives
Keep Loggers as they are. For new features, we would have to introduce special new hook just for the logger.
cc @Borda @awaelchli @edward-io @ananthsub @rohitgr7 @kamil-kaczmarek @Raalsky @Blaizzy @daniellepintz