Fixing question-answering with long contexts 
Author: Narsil
Pull request: yes

Fixes #13811

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Comments:
Having a similar issue where I used a QA model from transformers, tweaked the model in label studio making some annotations and then tried to load the model back again. The model is pulling out the correct answers but seemingly hadnle_impossible_answers isn't working because it gives an answer for every questions even when the question is irrelevant.. What's even weirder is that it doesn't do this in label studio's interface so seemingly this handle_impossible_answers is working on that side. My contexts I pass through the pipeline are also quite long

Made sure I have the most up to date transformers model, and the models were saved out with save_pretrained

```
model_path = "PATH"
model = AutoModelForQuestionAnswering.from_pretrained(model_path)
tokenizer = AutoTokenizer.from_pretrained(model_path) 
QA = pipeline('question-answering', model=model, tokenizer=tokenizer)

# Other code...
answers = QA(question=questions, context=context, top_k=1, max_answer_len=32, handle_impossible_answer=True)
```

The answers that I get out of here have an answer for every question, even when it doesn't need to be answered. I've tried loading the models multiple ways - the only way I got handle_impossible_answers to work as intended is if the tokenizer wasn't the AutoTokenizer that I had used here. But then the answers it gave me were complete garbage. Anybody else run into this issue with AutoTokenizer??





The only was I can think the tokenizer could be involved would be if it doesn't include EOS/BOS.

For null answer to come out, the score has to be the highest when the start and end logits are the first one (which should be a BOS/CLS token). Did you finetune your model on such null answers ? Maybe the finetuning just made that output impossible for the model to produce ?