Fix pytorch seq2seq qa (#19258)
* fixed typo for SQuAD
* Fixed the preprocess_validation_function function for the labels to reflect the remaining truncated instances
* Rolled back the trainer_seq2seq_qa.py for UnboundLocalError: local variable 'metrics' referenced before assignment
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
