I cannot get batches of data in pipelines

I have the following code to run text summarisation:

First I create the pipeline

summariser = pipeline("summarization", device=0,
                      model="sshleifer/distilbart-cnn-12-6", revision="a4f8f3e",
                      max_length=max_tokens, batch_size=batch_size)

And then I run it (here I break after one cycle to check the size of the output)

summaries = []
for out in tqdm(summariser(KeyDataset(ds_to_shorten, "description"), batch_size=batch_size)):

Now, len(out) is always 1, meaning that it is not returning batches of data, am I right?

What am I doing wrong?