question-answering pipeline error : too many values to unpack (expected 2)
For some `question-answering` models the pipleline encounters extra tuples from the `.model()` call, where we must have exactly 2.

> This stub results in the following error:
```python
from transformers.pipelines import pipeline

model_name = "mrm8488/bert-uncased-finetuned-qnli"

nlp = pipeline("question-answering", model=model_name, tokenizer=model_name)
QA_input = {
    "question": "Why is model conversion important?",
    "context": "The option to convert models between FARM and transformers gives freedom to the user and let people easily switch between frameworks.",
}
res = nlp(QA_input)
```

> error message
```bash
Traceback (most recent call last):
  File "test3.py", line 10, in <module>
    res = nlp(QA_input)
  File "/opt/conda/lib/python3.7/site-packages/transformers/pipelines.py", line 1010, in __call__
    start, end = self.model(**fw_args)
ValueError: too many values to unpack (expected 2)
```

> env
```text
- `transformers` version: 2.8.0
- Platform: Linux-4.9.184-linuxkit-x86_64-with-debian-buster-sid
- Python version: 3.7.4
- PyTorch version (GPU?): 1.4.0 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: nope
- Using distributed or parallel set-up in script?: nope
```
hmmm, `mrm8488/bert-uncased-finetuned-qnli` is a sequence classification model, not a QA model.

You probably get warnings while loading it in a QA Pipeline.

Does this happen with other (QA) models?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
