question-answering examples bug in pipelines document
Regarding
https://github.com/huggingface/transformers/blob/96f57c9ccb6363623005fb3f05166dfd7acb3f53/src/transformers/pipelines.py#L1739

it will cause a reproducible bug:

```python
from transformers import pipeline
nlp_qa = pipeline('question-answering', model='distilbert-base-cased-distilled-squad', tokenizer='bert-base-cased')
nlp_qa(context='Hugging Face is a French company based in New-York.', question='Where is based Hugging Face ?')

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/my_name/Library/Python/3.7/lib/python/site-packages/transformers/pipelines.py", line 1188, in __call__
    start, end = self.model(**fw_args)
  File "/usr/local/lib/python3.7/site-packages/torch/nn/modules/module.py", line 550, in __call__
    result = self.forward(*input, **kwargs)
TypeError: forward() got an unexpected keyword argument 'token_type_ids'
```

but it should use `tokenizer='distilbert-base-cased'`

```python
from transformers import pipeline
nlp_qa = pipeline('question-answering', model='distilbert-base-cased-distilled-squad', tokenizer='distilbert-base-cased')
nlp_qa(context='Hugging Face is a French company based in New-York.', question='Where is based Hugging Face ?')

{'score': 0.9632966867654424, 'start': 42, 'end': 50, 'answer': 'New-York.'}
```
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