K-fold cross-validation
Performs a k-fold cross-validation.

Inputs
Events
On each fold
Triggered K times, one for each fold.
Event outputs:
- 
    
Training — Tensor
Training data, of size
Data.size * (k - 1) / k. - 
    
Testing — Tensor
Testing data, of size
Data.size * 1 / k. 
On finish
Triggered after all folds have been processed.