Fix pytorch seq2seq qa
I created this pull request based on our discussion https://github.com/huggingface/transformers/issues/15398#issuecomment-1262926868

I also changed the typo in the README for the SQuADs feature. Then I also saw that the [trainer_seq2seq_qa.py](https://github.com/huggingface/transformers/blob/main/examples/pytorch/question-answering/trainer_seq2seq_qa.py) changed since I had it locally. And it caused me an issue, below you can see the slight adjustment, basically similar to how it was before.

I had been using the [run_seq2seq_qa.py](https://github.com/huggingface/transformers/blob/main/examples/pytorch/question-answering/run_seq2seq_qa.py) file with local text files, slightly adjusting the code, so I have tried with a `--do_predict` on my local setting, but not for an online dataset.


_The documentation is not available anymore as the PR was closed or merged._<|||||>Gently pinging @sgugger for final approval.

`test_run_swag_no_trainer` failed but it is not related to this PR :thinking: 