Cross-validation
🚀 Feature

Motivation
I believe cross-validation directly implemented as part of pytorch-lightning could help a lot of users!

Pitch
Trainer.train could have a method cross-validation with parameters similar to train and a parameter for cross-validation-split. This is a nice feature to have, especially when we have small model/dataset!

Alternatives

Additional context
We can keep sklearn as a reference for this https://scikit-learn.org/stable/modules/cross_validation.html