Problem using add_special_tokens
Author: hhou435
Pull request: no

## Environment info
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- `transformers` version:3.4.0
- Platform:windows
- Python version:3.7.0
- PyTorch version (GPU?):1.7.0
- Tensorflow version (GPU?):2.4.1
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

### Who can help
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @jplu

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- nlp datasets: [different repo](https://github.com/huggingface/nlp)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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@n1t0, @LysandreJik
## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Hi,I want to add some special tokens to bert tokenizer and these tokens are already part of the vocabulary.
![image](https://user-images.githubusercontent.com/59219579/109287775-feade400-785e-11eb-9fae-cf3873e5f715.png)
So i use `add_special_tokens`
```
from transformers import BertTokenizer
tokenizer = BertTokenizer.from_pretrained('t5')
tokenizer.add_special_tokens({'extra_id_0':'[extra_id_0]'},)
```
But something went wrong
```
Traceback (most recent call last):
  File "E:/github/Update_model/update_t5.py", line 10, in <module>
    tokenizer.add_special_tokens({'extra_id_0':'[extra_id_0]'},)
  File "D:\Anaconda\envs\hc\lib\site-packages\transformers\tokenization_utils_base.py", line 948, in add_special_tokens
    assert key in self.SPECIAL_TOKENS_ATTRIBUTES, f"Key {key} is not a special token"
AssertionError: Key extra_id_0 is not a special token
```
And I have another question.
I want to show the special tokens in the Hosted inference API when generate texts.
But I dont know how to do this.
Thanks!
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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->


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