When is `on_validation_epoch_start` / `on_validation_epoch_end` being called?
❓ Questions and Help

What is your question?
When is on_validation_epoch_start / on_validation_epoch_end being called?
It there a doc that explains the order of the callback function being called?
I need a callback function that will be called at the end of every validation epoch.
I read the callback docs and I think on_validation_epoch_end is the target function I need to override. But after my trial and error, I found that it is on_validation_end that meets my needs.
What have you tried?
And I found that on the latest version, nobody will call on_validation_epoch_start except the function on_validation_epoch_start itself in "callback_hook.py". Is it means that on_validation_epoch_start will never be called?
~/pytorch-lightning$ grep -rH "on_validation_epoch_start"
pytorch_lightning/core/hooks.py:    def on_validation_epoch_start(self) -> None:
pytorch_lightning/callbacks/base.py:    def on_validation_epoch_start(self, trainer, pl_module):
pytorch_lightning/trainer/callback_hook.py:    def on_validation_epoch_start(self):
pytorch_lightning/trainer/callback_hook.py:            callback.on_validation_epoch_start(self, self.get_model())

What's your environment?

OS: Linux
Packaging: pip
Version: 0.9.0