Thanks for the link! But how can you use a custom dataset on that apporach?
They use a 
dataset. In the official documentation they are using a single csv file for training: pd.read_csv(path/'cnndm_sample.csv'), but shouldn’t you use at least train and validation?
Edit:
The answer is:
dblock = DataBlock(blocks=blocks, get_x=ColReader('article'), get_y=ColReader('highlights'), splitter=RandomSplitter())
They split the single csv file into two dataloader object.