Apply validation on-demand

[borkowski1110]

1: Add validation only if selected!!!
a. If validation path is present, perform validation.
b. Else if validation split is set (some %) split the training set
then add validation_split to the fit() method inside the generator, no validation generator in this case.
c. If there is no path nor split do not run the validation at all.
5. Testing set and its statistics, to ponder!