How can I mute the info from BertTokenizerFast.encode_plus
# 🐛 Bug

## Information

Model I am using: 
Bert

Language I am using the model on (English, Chinese ...):
Chinese

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)
```python
tokenizer.encode_plus(...)
```
Every time I run this line, the info "Fast tokenizers add special tokens by default. To remove special tokens, please specify`add_special_tokens=False` during the initialisation rather than when calling `encode`,`encode_plus` or `batch_encode_plus`." will come out.

Even though I have already set the add_special_tokens to False
```python
tokenizer = BertTokenizerFast.from_pretrained(model_path, add_special_tokens = False)
```
The info can be very annoying when I run an iteration loop. How can I mute this info.

## Environment info
     
- `transformers` version: 2.8.0
- Platform: Linux-4.4.0-87-generic-x86_64-with-debian-stretch-sid
- Python version: 3.6.9
- PyTorch version (GPU?): 1.4.0+cu100 (True)
- Tensorflow version (GPU?): 2.0.0 (True)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no


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.
