Tokenizer.save_pretrained fails when add_special_tokens=True|False
Author: shuttie
Pull request: no

### System Info

transformers-4.34
python-3.11

### 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

```python
from transformers import AutoTokenizer

tok = AutoTokenizer.from_pretrained("mistralai/Mistral-7B-v0.1", add_special_tokens=True)
tok.save_pretrained("out")
```

The snippet:
* works well on `add_special_tokens=` being present, absent, True/False on 4.33 and below
* works well when `add_special_tokens=` is not added to the list of tokenizer parameters on 4.34+
* fails when `add_special_tokens=` is present in parameters (with both True/False values) on 4.34+  with the following error:

```
Traceback (most recent call last):
  File "/home/shutty/private/code/savepbug/test.py", line 4, in <module>
    tok.save_pretrained("tokenz")
  File "/home/shutty/private/code/savepbug/.venv/lib/python3.11/site-packages/transformers/tokenization_utils_base.py", line 2435, in save_pretrained
    out_str = json.dumps(tokenizer_config, indent=2, sort_keys=True, ensure_ascii=False) + "\n"
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
          ^^^^^^^^^^^
  File "/usr/lib/python3.11/json/encoder.py", line 202, in encode
    chunks = list(chunks)
             ^^^^^^^^^^^^
  File "/usr/lib/python3.11/json/encoder.py", line 432, in _iterencode
    yield from _iterencode_dict(o, _current_indent_level)
  File "/usr/lib/python3.11/json/encoder.py", line 406, in _iterencode_dict
    yield from chunks
  File "/usr/lib/python3.11/json/encoder.py", line 439, in _iterencode
    o = _default(o)
        ^^^^^^^^^^^
  File "/usr/lib/python3.11/json/encoder.py", line 180, in default
    raise TypeError(f'Object of type {o.__class__.__name__} '
TypeError: Object of type method is not JSON serializable
```

The issue happens on any tokenizer, not only on LLama one. I can confirm it failing the same way on `bert-base-uncased`

If you go to the `tokenization_utils_base` and dump the tokenizer_config just before the `json.dumps`, you may see that `add_special_tokens` surprizingly became a method, and not a bool:

```
{'clean_up_tokenization_spaces': False, 'unk_token': '<unk>', 'bos_token': '
', 'eos_token': '
', 'add_bos_token': True, 
'add_eos_token': False, 'use_default_system_prompt': False, 'additional_special_tokens': [], 'legacy': True, 
'model_max_length': 1000000000000000019884624838656, 'pad_token': None, 'sp_model_kwargs': {}, 
'spaces_between_special_tokens': False, 
'add_special_tokens': <bound method SpecialTokensMixin.add_special_tokens of LlamaTokenizerFast(name_or_path='mistralai/Mistral-7B-v0.1', vocab_size=32000, model_max_length=1000000000000000019884624838656, is_fast=True, 
padding_side='left', truncation_side='right', special_tokens={'bos_token': '
', 'eos_token': '
', 'unk_token': '<unk>'}, clean_up_tokenization_spaces=False),  added_tokens_decoder={
        0: AddedToken("<unk>", rstrip=False, lstrip=False, single_word=False, normalized=False, special=True),
        1: AddedToken("
", rstrip=False, lstrip=False, single_word=False, normalized=False, special=True),
        2: AddedToken("
", rstrip=False, lstrip=False, single_word=False, normalized=False, special=True),
}>, 'added_tokens_decoder': {0: {'content': '<unk>', 'single_word': False, 'lstrip': False, 'rstrip': False, 
'normalized': False, 'special': True}, 1: {'content': '
', 'single_word': False, 'lstrip': False, 'rstrip': False,
 'normalized': False, 'special': True}, 2: {'content': '
', 'single_word': False, 'lstrip': False, 'rstrip': False,
 'normalized': False, 'special': True}}, 'tokenizer_class': 'LlamaTokenizer'}
```

My feeling that the issue is related to the https://github.com/huggingface/transformers/pull/23909 PR which refactored a lot of tokenizer internals, so in the current version:
* `add_special_tokens` is a part of kwargs passed to the tokenizer
* there is also a method `SpecialTokensMixin.add_special_tokens` having the same name
* when everything is being joined together before `json.dumps`, the method is being serialized instead of the kwargs parameter.

### Expected behavior

Not crashing with `TypeError: Object of type method is not JSON serializable` as in was pre https://github.com/huggingface/transformers/pull/23909 in 4.33.

Comments:
It is indeed related to that PR, but it is also related to the fact that `add_special_tokens` even if saved, is not used when doing an encode pass. Thus it's better to error out than save it IMO as it won't be checked when encoding. I'll have a look at the PR 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

Could you please let me know how to overcome this issue or if this is addressed in any of the latest releases of transformers library. This seems have further impact with save_checkpoints in the Trainer of the SFTTrainer and causing it to fail.

Hey a PR was open  see #31233