Register a custom tokenizer with AutoTokenizer
### System Info

(Possible duplicate: #10256)
I have written a custom tokenizer that builds on top of `BertTokenizer` (returns one extra list of ids that will later be embedded in a custom model). I have pushed it to Hub as well. Now, how can I allow others to use it? The code for the tokenizer is uploaded to Hub along with the code for the model (they are in the same file), but since I cannot register the tokenizer with `AutoTokenizer` like I can do for models (`CustomModel.register_for_auto_class("AutoModel")`), others cannot load this tokenizer, and hence use the model.

Is there a workaround for this?

Version: 4.27.4

### Who can help?

@ArthurZucker @younesbelkada

### 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

The code for both the tokenizer and model can be found here: https://huggingface.co/mcgill-babylm/bert_ds10M_np512_nh2_nl2_hs128_postags_ungrouped/blob/main/pos_bert.py

I am able to load the model with no problems since I push it after registering it as follows
```
BertForMaskedLMWithPOSEmb.register_for_auto_class("AutoModel")
BertForMaskedLMWithPOSEmb.register_for_auto_class("AutoModelForMaskedLM")
```

### Expected behavior

I should be able to register custom tokenizers with `AutoTokenizer` (which might be a new feature request) or work around it somehow to allow other users to use a custom tokenizer.
You can have a look at the [documentation here](https://huggingface.co/docs/transformers/custom_models) but this is already supported. Just do `CustomTokenizer.register_for_auto_class()` like for the models.<|||||>Duh! I was doing this for the models but didn't make the connection to the tokenizer. Thanks @sgugger!

For someone looking for the complete answer in the future:
```
CustomTokenizer.register_for_auto_class("AutoTokenizer")
```