squad_convert_example_to_features does not work with CamembertTokenizer
# 🐛 Bug

## Information

Model I am using : CamemBERT
Language I am using the model on : French

The problem arises when using:
* [*] my own modified scripts: (give details below)

The tasks I am working on is:
* [*] an official GLUE/SQUaD task: SQUaD

## To reproduce

Steps to reproduce the behavior:
1 - Copy paste this and run it
```python
from transformers import CamembertTokenizer, SquadExample, squad_convert_examples_to_features

tokenizer = CamembertTokenizer.from_pretrained('camembert-base')
example = SquadExample(
    'example_id',
    "Q",
    "C D E F G H",
    "D",
    2,
    "title"
)
features, _ = squad_convert_examples_to_features(
    examples=[
        example
    ],
    tokenizer=tokenizer,
    max_seq_length=30,
    doc_stride=128,
    max_query_length=128,
    is_training=True,
    return_dataset="pt",
    threads=1,
)

tokens = tokenizer.convert_ids_to_tokens(tokenizer.encode(example.question_text, example.context_text))
doc_token = example.doc_tokens

print({tokens[k]: doc_token[v] for k, v in features[0].token_to_orig_map.items()})
# Outputs 
# {'</s>': 'C', '▁C': 'D', '▁D': 'E', '▁E': 'F', '▁F': 'G', '▁G': 'H'}
# Should be
# {'▁C': 'C', '▁D': 'D', '▁E': 'E', '▁F': 'F', '▁G': 'G', '▁H': 'H'}
```

## Expected behavior

The resulting features mapping is shifted by one when using the CamembertTokenizer.

This seems to be caused by a weird check in the method `squad_convert_example_to_features`.  (`if "roberta" in str(type(tokenizer))`) is evaluated to False when using the CamembertTokenizer (which is adapted from RobertaTokenizer)

When I patch the line `if "roberta" in str(type(tokenizer))` by `if "roberta" in str(type(tokenizer)) or "camembert" in str(type(tokenizer))`, I get the expected behavior. 

I do not really know what would be the best way to handle this problem.

## Environment info

- `transformers` version: 2.4.1
- Platform: MacOS
- Python version: 3.7.6
- PyTorch version : 1.4
- Tensorflow version : None
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

Solved by #2746 