Error arises when using pipeline with community model
# 🐛 Bug

## Information

Model I am using is: `mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es`

Language I am using the model on: Spanish

The problem arises when using:
* [ ] the official example scripts:
* [x] my own modified scripts:

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task:
* [x] my own task or dataset:

Steps to reproduce the behavior:

```python
from transformers import *

# Build a pipeline for QA
nlp = pipeline('question-answering', model='mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es',
               tokenizer='mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es')

nlp(
    {
      'question': 'que queso es?',
      'context': 'Se utilizo en el dia de hoy un queso Emmental'
    }
)
```

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This was working two days ago.

<details>
  <summary>Error log</summary>
  
  ```html
convert squad examples to features:   0%|          | 0/1 [00:00<?, ?it/s]WARNING:transformers.tokenization_utils:Disabled padding because no padding token set (pad_token: [PAD], pad_token_id: 1).
To remove this error, you can add a new pad token and then resize model embedding:
	tokenizer.pad_token = '<PAD>'
	model.resize_token_embeddings(len(tokenizer))
---------------------------------------------------------------------------
RemoteTraceback                           Traceback (most recent call last)
RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 119, in worker
    result = (True, func(*args, **kwds))
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 44, in mapstar
    return list(map(*args))
  File "/usr/local/lib/python3.6/dist-packages/transformers/data/processors/squad.py", line 141, in squad_convert_example_to_features
    truncation_strategy="only_second" if tokenizer.padding_side == "right" else "only_first",
  File "/usr/local/lib/python3.6/dist-packages/transformers/tokenization_utils.py", line 1796, in encode_plus
    **kwargs,
  File "/usr/local/lib/python3.6/dist-packages/transformers/tokenization_utils.py", line 1722, in batch_encode_plus
    tokens = self._tokenizer.encode(*batch_text_or_text_pairs[0])
  File "/usr/local/lib/python3.6/dist-packages/tokenizers/implementations/base_tokenizer.py", line 141, in encode
    return self._tokenizer.encode(sequence, pair)
TypeError
"""

The above exception was the direct cause of the following exception:

TypeError                                 Traceback (most recent call last)
<ipython-input-21-df466dea770c> in <module>()
      8     nlp({
      9     'question': question,
---> 10     'context': context
     11     })
     12 )

11 frames
/usr/local/lib/python3.6/dist-packages/tokenizers/implementations/base_tokenizer.py in encode()
    139             An Encoding
    140         """
--> 141         return self._tokenizer.encode(sequence, pair)
    142 
    143     def encode_batch(self, sequences: List[Union[str, Tuple[str, str]]]) -> List[Encoding]:

TypeError: 
  ```
</details>

## Environment info
<!-- You can run the command `python transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.5.0
- Python version:3.6.9
- Torch version (GPU?): 1.4.0, running on CPU

Hi @ankandrew, 

Thanks for reporting the issue. Effectively, the QA pipeline is not compatible with fast tokenizers for technical reasons (and I'm currently working on a fix for this).

As a workaround for now, you can disable fast tokenizers when allocating the pipeline:

```python
nlp = pipeline(
    'question-answering', 
    model='mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es',
    tokenizer=(
        'mrm8488/distill-bert-base-spanish-wwm-cased-finetuned-spa-squad2-es',  
        {"use_fast": False}
    )
)

nlp(
    {
        'question': 'que queso es?',
        'context': 'Se utilizo en el dia de hoy un queso Emmental'
    }
)
> {'score': 0.36319364208159755, 'start': 31, 'end': 44, 'answer': 'queso Emmental'}
```<|||||>Also cc'ing @mrm8488 for information while it's in the process of being fixed<|||||>Thank for the information!<|||||>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.
