Saving checkpoints on enwiki8 model.

Hi and thank you for this wonderful package.

Seeing the code for the enwiki8 transformer model, you don't seem to save anywhere the model or the weights, for latter use. As so, the model cannot later be used for generation.

Can you maybe sketch a checkpointing strategy (what needs to be saved and how) or there is for not doing so in the first place? 

Thanks in advance!
