

TITLE
tokenizer return_special_tokens does not work correctly with custom special tokens

COMMENTS
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BODY
The tokenizer's returned `special_tokens_mask` does not take into account the newly added `special_tokens`.

### System Info

```shell
- `transformers` version: 4.18.0
- Platform: Linux-5.4.0-92-generic-x86_64-with-glibc2.31
- Python version: 3.9.12
- Huggingface_hub version: 0.5.1
- PyTorch version (GPU?): 1.11.0 (False)
```

### Who can help?

@LysandreJik 

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```python
import transformers
print(transformers.__version__)
tokenizer = transformers.AutoTokenizer.from_pretrained('roberta-base')

special_tokens_dict = {"additional_special_tokens": ["<test1>", "<test2>"]}
tokenizer.add_special_tokens(special_tokens_dict)

processed = tokenizer("this <test1> that <test2> this", return_special_tokens_mask=True)
tokens = tokenizer.convert_ids_to_tokens(processed.input_ids)

for i in range(len(processed.input_ids)):
    print(f"{processed.input_ids[i]}\t{tokens[i]}\t{processed.special_tokens_mask[i]}")
```

### Expected behavior

```shell
Returned output:

0       <s>     1
9226    this    0
1437    Ġ       0
50265   <test1> 0
14      Ġthat   0
1437    Ġ       0
50266   <test2> 0
42      Ġthis   0
2       </s>    1


Expected output:


0       <s>     1
9226    this    0
1437    Ġ       0
50265   <test1> 1
14      Ġthat   0
1437    Ġ       0
50266   <test2> 1
42      Ġthis   0
2       </s>    1

