The pre-processing step only helped but not solve this problem. This specific splits-method returns NLIDatasets with a filter attribute, kicking out a specific kind of label ( ‘-’ ). The right way to do would possibly involve using TabularDataset and defininig fields similar to the NLIDatasets ones, but since ‘.jsonl’ is not supported and an easy hacky way seems to work, I will use this version for now.
class_labels = {'contradiction': 0, 'neutral': '-', 'entailment': 1, '-': '-'}
label_tokenizer = lambda x: class_labels[x]
label_field = data.Field(sequential=False, batch_first=True, preprocessing=label_tokenizer)

https://pytorch.org/text/_modules/torchtext/datasets/nli.html 3