Llama fast tokenizer `train_new_from_iterator` returns `TypeError: 'NoneType' object is not subscriptable`
### System Info

accelerate==0.18.0
aiohttp==3.8.4
aiosignal==1.3.1
anyio==3.6.2
argon2-cffi==21.3.0
argon2-cffi-bindings==21.2.0
arrow==1.2.3
asttokens==2.2.1
async-timeout==4.0.2
attrs==23.1.0
backcall==0.2.0
beautifulsoup4==4.12.2
bitsandbytes==0.38.1
bleach==6.0.0
certifi==2022.12.7
cffi==1.15.1
charset-normalizer==3.1.0
cmake==3.26.3
comm==0.1.3
datasets==2.11.0
debugpy==1.6.7
decorator==5.1.1
defusedxml==0.7.1
dill==0.3.6
evaluate==0.4.0
executing==1.2.0
fastjsonschema==2.16.3
filelock==3.12.0
fqdn==1.5.1
frozenlist==1.3.3
fsspec==2023.4.0
huggingface-hub==0.13.4
idna==3.4
importlib-metadata==6.5.0
importlib-resources==5.12.0
ipykernel==6.22.0
ipython==8.12.0
ipython-genutils==0.2.0
isoduration==20.11.0
jedi==0.18.2
Jinja2==3.1.2
jsonpointer==2.3
jsonschema==4.17.3
jupyter-events==0.6.3
jupyter_client==8.2.0
jupyter_core==5.3.0
jupyter_server==2.5.0
jupyter_server_terminals==0.4.4
jupyterlab-pygments==0.2.2
lit==16.0.1
MarkupSafe==2.1.2
matplotlib-inline==0.1.6
mistune==2.0.5
mpmath==1.3.0
multidict==6.0.4
multiprocess==0.70.14
nbclassic==0.5.5
nbclient==0.7.3
nbconvert==7.3.1
nbformat==5.8.0
nest-asyncio==1.5.6
networkx==3.1
notebook==6.5.4
notebook_shim==0.2.2
numpy==1.24.2
nvidia-cublas-cu11==11.10.3.66
nvidia-cuda-cupti-cu11==11.7.101
nvidia-cuda-nvrtc-cu11==11.7.99
nvidia-cuda-runtime-cu11==11.7.99
nvidia-cudnn-cu11==8.5.0.96
nvidia-cufft-cu11==10.9.0.58
nvidia-curand-cu11==10.2.10.91
nvidia-cusolver-cu11==11.4.0.1
nvidia-cusparse-cu11==11.7.4.91
nvidia-nccl-cu11==2.14.3
nvidia-nvtx-cu11==11.7.91
packaging==23.1
pandas==2.0.0
pandocfilters==1.5.0
parso==0.8.3
pexpect==4.8.0
pickleshare==0.7.5
pkgutil_resolve_name==1.3.10
platformdirs==3.2.0
prometheus-client==0.16.0
prompt-toolkit==3.0.38
protobuf==3.20.0
psutil==5.9.5
ptyprocess==0.7.0
pure-eval==0.2.2
pyarrow==11.0.0
pycparser==2.21
Pygments==2.15.1
pyrsistent==0.19.3
python-dateutil==2.8.2
python-dotenv==1.0.0
python-json-logger==2.0.7
pytz==2023.3
PyYAML==6.0
pyzmq==25.0.2
regex==2023.3.23
requests==2.28.2
responses==0.18.0
rfc3339-validator==0.1.4
rfc3986-validator==0.1.1
Send2Trash==1.8.0
sentencepiece==0.1.98
six==1.16.0
sniffio==1.3.0
soupsieve==2.4.1
stack-data==0.6.2
sympy==1.11.1
terminado==0.17.1
tinycss2==1.2.1
tokenizers==0.13.3
torch==2.0.0
tornado==6.3
tqdm==4.65.0
traitlets==5.9.0
-e git+https://github.com/huggingface/transformers.git@474bf508dfe0d46fc38585a1bb793e5ba74fddfd#egg=transformers
triton==2.0.0
typing_extensions==4.5.0
tzdata==2023.3
uri-template==1.2.0
urllib3==1.26.15
wcwidth==0.2.6
webcolors==1.13
webencodings==0.5.1
websocket-client==1.5.1
xxhash==3.2.0
yarl==1.8.2
zipp==3.15.0


### Who can help?

@ArthurZucker , @Narsil 

### Information

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

### Tasks

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

### Reproduction

1. Convert llama weights to hf format
```
python src/transformers/models/llama/convert_llama_weights_to_hf.py \
    --input_dir /path/to/downloaded/llama/weights --model_size tokenizer_only --output_dir /output/path
```

2.  Train new tokenizer from old.
```
from transformers import AutoTokenizer
old_tokenizer = AutoTokenizer.from_pretrained(/output/path)

old_tokenizer.train_new_from_iterator(["I love huggingface!"], 50)
```

### Expected behavior

## Behavior
I ran into the error:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[3], line 5
      3 old_tokenizer = AutoTokenizer.from_pretrained(PATH_TO_LLAMA_DIR,)
----> 5 old_tokenizer.train_new_from_iterator(["I love huggingface!"], 50)

File ~/transformers/src/transformers/tokenization_utils_fast.py:709, in PreTrainedTokenizerFast.train_new_from_iterator(self, text_iterator, vocab_size, length, new_special_tokens, special_tokens_map, **kwargs)
    [707](file:///home/jovyan/transformers/src/transformers/tokenization_utils_fast.py?line=706) if tokenizer_json["model"]["type"] == "Unigram" and unk_token is not None:
    [708](file:///home/jovyan/transformers/src/transformers/tokenization_utils_fast.py?line=707)     kwargs["unk_token"] = unk_token
--> [709](file:///home/jovyan/transformers/src/transformers/tokenization_utils_fast.py?line=708) if tokenizer_json["pre_tokenizer"]["type"] == "ByteLevel":
    [710](file:///home/jovyan/transformers/src/transformers/tokenization_utils_fast.py?line=709)     kwargs["initial_alphabet"] = pre_tokenizers_fast.ByteLevel.alphabet()
    [712](file:///home/jovyan/transformers/src/transformers/tokenization_utils_fast.py?line=711) trainer_class = MODEL_TO_TRAINER_MAPPING[tokenizer_json["model"]["type"]]

TypeError: 'NoneType' object is not subscriptable
```

## Analysis
Inspecting my `tokenizer.json` file ([tokenizer.zip](https://github.com/huggingface/transformers/files/11279412/tokenizer.zip)), I realised my `"pre_tokenizer": null,`  which led to the error.  

I'm not sure if it helps, but I had issue converting the llama weights to hf format (step 1) due to the protobuf version bug described [here](https://github.com/huggingface/transformers/issues/21128). I fixed it by downgrading my protobuf to version 3.20. 
Same problem here. The code appears to be looking for a ByteLevel pretokenizer, but the json.load(_tokenizer) at line 644 of tokenization_utils_fast.py is initializing one with pretokenizer equal to None<|||||>Hey! Thanks for reporting! I can reproduce this, indeed it's bug will investigate<|||||>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.<|||||>Should have been fixed by #22959 