Hey guys,
I have fixed it by providing the iterator with a way to compare the examples e.g.
data.Iterator.splits(
        (train_data, eval_data),
        batch_size=batch_size, ...,  sort_key=lambda x: len(x.text))

Here is the documentation in the source code: https://github.com/pytorch/text/blob/2ce827edd10a92de83a2578cea9cf1b0e3b3f1af/torchtext/data/iterator.py#L27 24