prefix = "summarize: "
def preprocess_function(examples):
     inputs = [prefix + doc for doc in examples["text"]]
     model_inputs = tokenizer(inputs, max_length=1024, truncation=True)

     labels = tokenizer(text_target=examples["summary"], max_length=128, truncation=True)
     model_inputs["labels"] = labels["input_ids"]
     return model_inputs

To apply the preprocessing function over the entire dataset, use 🤗 Datasets [~datasets.Dataset.map] method.