Add on_validation_model_train/eval to Callback API as well #20441
Description
Description & Motivation
Today on_validation_model_train/eval (similar to on_test_xxx) is only available in LightningModule. It would be helpful to open up this interface to callback as well especially as callbacks are much more pluggable. This will be very helpful for implementing ScheduleFree optimizers which has been proved to be very convergence efficient: https://github.com/facebookresearch/schedule_free/tree/main/schedulefree
I'd be happy to draft a PR if this idea is approved.
Pitch
Add on_validation_model_train/eval (and those for test) to callback API
Alternatives
No response
Additional context
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