Why do training scripts for fine-tuning BERT-based models on SQuAD (e.g., this one from google or this one from HuggingFace,  use set a maximum length of 384 (by default) for input sequences even though the models can handle inputs of length up to 512? (This maximum length refers to the combined length of the question and context, right? Regardless, the questions in the SQuAD dataset typically have length significantly less than 128.)