question answering pipeline throws error when handle_impossible_answer=True
## Environment info
- `transformers` version: 4.12.3
- Platform: Linux-5.4.104+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.12
- PyTorch version (GPU?): 1.9.0+cu111 (True)
- Tensorflow version (GPU?): 2.6.0 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help

Pipelines: @Narsil 

## Information

The problem arises when using:
my own modified scripts: (give details below)  
with any model

## To reproduce

See an example here https://github.com/jsalbr/PythonIntro/blob/master/Transformers_Issue.ipynb
or directly in colab: https://colab.research.google.com/github/jsalbr/PythonIntro/blob/master/Transformers_Issue.ipynb

or briefly here
```python
from transformers import pipeline

model_name = "deepset/minilm-uncased-squad2"
qa = pipeline("question-answering", model=model_name,
              tokenizer=model_name, device=0)

question = "How is the weather?"
context = """Washington is the capital of the United States."""
# throws exception in transformers 4.12.3
answer = qa(question, context, handle_impossible_answer=True)
answer
```

Error message:
```
/usr/local/lib/python3.7/dist-packages/transformers/pipelines/question_answering.py in postprocess(self, model_outputs, top_k, handle_impossible_answer, max_answer_len)
    407 
    408             if handle_impossible_answer:
--> 409                 min_null_score = min(min_null_score, (start_[0] * end_[0]).item())
    410 
    411             # Mask CLS

ValueError: can only convert an array of size 1 to a Python scalar
```

## Expected behavior

No exception ;-)

I patched it this way, and it seems to work (though probably not correct):

```patch
409c409
<                 min_null_score = min(min_null_score, (start_[0] * end_[0]).item())
---
>                 min_null_score = min(min_null_score, (np.max(start_[0] * end_[0])).item())
```

This has been (hopefully) fixed in master.

Can you confirm: https://github.com/huggingface/transformers/pull/14279 ?<|||||>Yes, no error anymore.