Allow self.log from on_validation_end
🚀 Feature
Currently, self.log is not supported in on_validation_end. I would like it to be.
Motivation
I want to compute some metric each time the validation loop runs, but I don't want it to be tied to on_validation_batch_end: i.e I want to do my own batching. I also want to self.log this so I can then use it as a key to monitor for checkpointing. Finally, the documentation claims:
Use the log() method to log from anywhere in a LightningModule and Callback except functions with batch_start in their names.

I don't see why we should prevent logging in this method, it just adds a surprising friction.
Pitch
Don't prevent logging in on_validation_end.
Alternatives

Additional context