Tokenization issue with RoBERTa and DistilRoBERTa.
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...):
RoBERTa (roberta-base), DistilRoBERTa (distilroberta-base)
Language I am using the model on (English, Chinese ...):
English
The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

I am trying to encode the embeddings for the sentences, and I found a tokenization issue with a certain (type of) sentence which ends with ").". I noticed that the tokenizer cannot tokenize ')' from '.' and further causes issues with the sentence length.

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

**Dataset: SemEval 2016 Task 5, SB1 EN-REST**
## To reproduce

Steps to reproduce the behavior:

See in the following codes:
```python
import torch
import numpy as np
from transformers import AutoModel, AutoTokenizer

text = '(Besides that there should be more restaurants like it around the city).'
for model_name in ['roberta-base', 'distilroberta-base']:
    tokenizer = AutoTokenizer.from_pretrained(model_name)
    model = AutoModel.from_pretrained(model_name)
    token_dict = tokenizer.encode_plus(text, None, return_tensors='pt')
    
    print('model_name: {}'.format(model_name))
    print("Token (str): {}".format(
        tokenizer.convert_ids_to_tokens(token_dict['input_ids'][0])))
    print("Token (int): {}".format(token_dict['input_ids']))
    print("Type: {}".format(
        token_dict['token_type_ids']))
    print('Output Embeddings: {}\n'.format(
        model(token_dict['input_ids'])[0].shape))
```

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## Expected behavior
Expected output:
```
model_name: roberta-base
Token (str): ['<s>', 'Ġ(', 'Besides', 'Ġthat', 'Ġthere', 'Ġshould', 'Ġbe', 'Ġmore', 'Ġrestaurants', 'Ġlike', 'Ġit', 'Ġaround', 'Ġthe', 'Ġcity', ')', 'Ġ.', '</s>']
Token (int): tensor([[    0,    36, 41107,    14,    89,   197,    28,    55,  4329,   101,
            24,   198,     5,   343,    43,   479,     2]])
Type: tensor([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]])
Output Embeddings: torch.Size([1, 17, 768])

model_name: distilroberta-base
Token (str): ['<s>', 'Ġ(', 'Besides', 'Ġthat', 'Ġthere', 'Ġshould', 'Ġbe', 'Ġmore', 'Ġrestaurants', 'Ġlike', 'Ġit', 'Ġaround', 'Ġthe', 'Ġcity', ')', 'Ġ.', '</s>']
Token (int): tensor([[    0,    36, 41107,    14,    89,   197,    28,    55,  4329,   101,
            24,   198,     5,   343,    43,   479,     2]])
Type: tensor([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]])
Output Embeddings: torch.Size([1, 17, 768])
```
<!-- A clear and concise description of what you would expect to happen. -->
Basically, the expected behavior is to tokenize ')' and '.' separately. ~~Furthermore, I am also curious about what these 'Ġ' characters are in the RoBERTa encoding? I checked the vocabulary and I found both the normal words and the words starting with this 'Ġ' character so I am a bit confused.~~

## 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: 2.5.1
- Platform: Windows-10-10.0.18362-SP0
- Python version: 3.7.6
- PyTorch version (GPU?): 1.4.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: False
- Using distributed or parallel set-up in script?: False

>Furthermore, I am also curious about what these 'Ġ' characters are in the RoBERTa encoding?

It's a feature of byte-level BPE (an encoded space character)
[Ref-bart-fairseq](https://github.com/pytorch/fairseq/issues/1716), [Ref-openai-gpt](https://github.com/openai/gpt-2/issues/80)<|||||>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.
