Bug in the question answering pipeline
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.0.2
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.6.0+cu101 (False)
- Tensorflow version (GPU?): 2.3.0 (False)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no

## Information
The bug appears since transformers 3.0.1 but not before.

Model I am using distilbert-base-cased-distilled-squad:

The problem arises when using:
* [ ] my own modified scripts: 
```

from transformers import pipeline
model = "distilbert-base-cased-distilled-squad"
qa_pipeline = pipeline(
    "question-answering",
    model=model,
    tokenizer=model,
)

instance = {
  "question": "what is your product?",
  "context": " is an amazing new platform that help businesses of students from BarIlan University that are enthusiastic about conversational AI. The difference between our Sprybot platform and other chat bots is that constructing chat bot is a long and hard process and with Sprybot you can do it quickly and eaily. You can construct chatbot using our platform just by feeding textual description of you business that contain any details important for costumers. The time it takes to create a bot using our platform is the time takes you to describe your business. In order to create Sprybot we used natural language processing and state of the art deep learning artificial intelligence. At the moment you cant buy our product because its still under construction. Sprybot can answer questions about your business but it can not talk about anything else other than the information was fed to it."
}
qa_pipeline(instance)

```


Notice: little changes in the context text make the bug to not show up
## To reproduce

Steps to reproduce the behavior:

1. [fully reproduced on google colab](https://colab.research.google.com/drive/1YqamXA6qq8xxWXhq6VqEA9clHsEVW7sh?usp=sharing)


```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-a5f26c48556d> in <module>()
      4 }
      5 
----> 6 qa_pipeline(instance)

1 frames
/usr/local/lib/python3.6/dist-packages/transformers/pipelines.py in __call__(self, *args, **kwargs)
   1314                         ),
   1315                     }
-> 1316                     for s, e, score in zip(starts, ends, scores)
   1317                 ]
   1318 

/usr/local/lib/python3.6/dist-packages/transformers/pipelines.py in <listcomp>(.0)
   1314                         ),
   1315                     }
-> 1316                     for s, e, score in zip(starts, ends, scores)
   1317                 ]
   1318 

KeyError: 0
```


## Expected behavior

get the qa pipline output with no errors

Hi! This bug was patched on `master`. Can you install from source and let me know if this fixes your issue? 

`pip install git+https://github.com/huggingface/transformers`<|||||>This fixed it! Thank you!