Tiktoken tokenizers are not pickable 
### Describe the bug

Since tiktoken tokenizer is not pickable, it is not possible to use it inside `dataset.map()` with multiprocessing enabled. However, you [made](https://github.com/huggingface/datasets/issues/5536) tiktoken's tokenizers pickable in `datasets==2.10.0` for caching. For some reason, this logic does not work in dataset processing and raises `TypeError: cannot pickle 'builtins.CoreBPE' object`


### Steps to reproduce the bug

```
from datasets import load_dataset
import tiktoken

dataset = load_dataset("stas/openwebtext-10k")

enc = tiktoken.get_encoding("gpt2")

tokenized = dataset.map(
    process,
    remove_columns=['text'],
    desc="tokenizing the OWT splits",
    num_proc=2,
)

def process(example):
        ids = enc.encode(example['text'])
        ids.append(enc.eot_token)
        out = {'ids': ids, 'len': len(ids)}
        return out
```

### Expected behavior

starts processing dataset 

### Environment info

- `datasets` version: 2.11.0
- Platform: Linux-5.15.0-1021-oracle-x86_64-with-glibc2.29
- Python version: 3.8.10
- Huggingface_hub version: 0.13.4
- PyArrow version: 9.0.0
- Pandas version: 2.0.0