[Bis] Adding new tokens while preserving tokenization of adjacent tokens
### System Info

* `transformers` version: 4.31
* Platform: Linux [...] 5.19.0-50-generic 50-Ubuntu  x86_64 GNU/Linux
* Python version: 3.10.12
* Huggingface_hub version: 0.16.4
* PyTorch version (GPU?): 2.0.1+cu118 (True)
* Using GPU in script?: No
* Using distributed or parallel set-up in script?: No

### Who can help?

@ArthurZucker

### Information

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

### Tasks

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

### Reproduction

This issue is related to [this HuggingFace post on the official forum](https://discuss.huggingface.co/t/adding-new-tokens-while-preserving-tokenization-of-adjacent-tokens/12604), hence the similar title, and to my knowledge, no answer was given as to whether this is the normal tokenizer behavior. I ran into the same problem as the original poster while trying to tokenize a sentence after adding new tokens: the adjacent tokens of the newly added ones aren't computed with their preceded escape symbol.

```py
>>> import transformers
>>> tok = transformers.RobertaTokenizer.from_pretrained("roberta-base")
>>> lotr_sent = 'Aragorn told Frodo to mind Lothlorien'
>>> tok.convert_ids_to_tokens(tok(lotr_sent)['input_ids'])
['<s>', 'Ar', 'ag', 'orn', 'Ġtold', 'ĠFro', 'do', 'Ġto', 'Ġmind', 'ĠL', 'oth', 'lor', 'ien', '</s>']
>>> tok.add_tokens(['Aragorn', 'Frodo', 'Lothlorien'])
3
>>> tok.convert_ids_to_tokens(tok(lotr_sent)['input_ids'])
['<s>', 'Aragorn', 'told', 'Frodo', 'to', 'Ġmind', 'Lothlorien', '</s>']
```

### Expected behavior

The tokens `told`, `Frodo`, `to` and `Lothlorien` should be preceded with a `Ġ` character if I am not mistaken ; e.g.:
```py
>>> import transformers
>>> tok = transformers.RobertaTokenizer.from_pretrained("roberta-base")
>>> lotr_sent = 'Aragorn told Frodo to mind Lothlorien'
>>> tok.convert_ids_to_tokens(tok(lotr_sent)['input_ids'])
['<s>', 'Ar', 'ag', 'orn', 'Ġtold', 'ĠFro', 'do', 'Ġto', 'Ġmind', 'ĠL', 'oth', 'lor', 'ien', '</s>']
>>> tok.add_tokens(['Aragorn', 'Frodo', 'Lothlorien'])
3
>>> tok.convert_ids_to_tokens(tok(lotr_sent)['input_ids'])
['<s>', 'Aragorn', 'Ġtold', 'ĠFrodo', 'Ġto', 'Ġmind', 'ĠLothlorien', '</s>']
```
Hey! This has already been answered, and is a duplicate of #14770. Will be fixed by #23909. 
