While this may look like a lot, you're only really interested in the text field. What's cool about language modeling tasks is you don't need labels (also known as an unsupervised task) because the next word is the label.
Preprocess

For masked language modeling, the next step is to load a DistilRoBERTa tokenizer to process the text subfield:

from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("distilbert/distilroberta-base")