ImageDataGenerator should be able to write per label

[Napolitain]

Right now, if we want to save augmented images to directory, we must do what is described below.

https://stackoverflow.com/questions/63769334/imagedatagenerator-keras-save-to-dir

ImageDataGenerator is able to read this :

datasets/
-- class A/
-- class B/
-- class C/

but can't write like this with save_to_dir option :

augmented_datasets/
-- class A/
-- class B/
-- class C/

It should be trivial to do with that last option in my opinion.