References to the *_epoch_end hooks in Lightning 2.0

[awaelchli]

📚 Documentation

There are a few references in the docs where *_epoch_end() is mentioned, mostly when resetting metrics manually. In Lightning 2.0, these hooks got changed.

training_epoch_end(self, outputs) -> on_train_epoch_end(self)
validation_epoch_end(self, outputs) -> on_validation_epoch_end(self)
etc.

Most references have already been updated, but a few leftovers exist:
https://torchmetrics.readthedocs.io/en/latest/pages/overview.html#metriccollection
https://torchmetrics.readthedocs.io/en/latest/pages/lightning.html#logging-torchmetrics

I can send a PR for this