The empty val set and test set

[JunXieFront]

Thanks for your tutotial of Attention is all you need, and I have a small question.I would really appreciate for an answer.Because the project only has datasets of training, why we need a val_loader in train.py.Should I download data for val set by myself？
val_loader = SequenceLoader( data_folder=data_folder, source_suffix='en', target_suffix='de', split='val', tokens_in_batch=tokens_in_batch )