Trying to implement "nielsr/luke-large" gives "KeyError: 'luke'"
## Environment info

- `transformers` version: 4.1.1
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.8.3
- PyTorch version (GPU?): 1.7.1+cpu (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@LysandreJik I guess, because it's an `AutoTokenizer`-related issue.

## Information

I'm trying to use an implementation of LUKE ([paper](https://arxiv.org/abs/2010.01057)) ([implementation](https://huggingface.co/nielsr/luke-large/tree/main)).

The problem arises when using:
* my own modified scripts

The task I am working on is: 
I don't think this is relevant.

## To reproduce

Steps to reproduce the behavior:

1. `from transformers import AutoTokenizer, AutoModel`
2. `tokenizer = AutoTokenizer.from_pretrained("nielsr/luke-large")`

Running gives the following error:

```
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-12-c1614eef2346> in <module>
      4 
----> 5 luke_tokenizer = AutoTokenizer.from_pretrained("nielsr/luke-large")
      6 

c:\...\venv\lib\site-packages\transformers\models\auto\tokenization_auto.py in from_pretrained(cls, pretrained_model_name_or_path, *inputs, **kwargs)
    343         config = kwargs.pop("config", None)
    344         if not isinstance(config, PretrainedConfig):
--> 345             config = AutoConfig.from_pretrained(pretrained_model_name_or_path, **kwargs)
    346 
    347         use_fast = kwargs.pop("use_fast", True)

c:\...\venv\lib\site-packages\transformers\models\auto\configuration_auto.py in from_pretrained(cls, pretrained_model_name_or_path, **kwargs)
    350 
    351         if "model_type" in config_dict:
--> 352             config_class = CONFIG_MAPPING[config_dict["model_type"]]
    353             return config_class.from_dict(config_dict, **kwargs)
    354         else:

KeyError: 'luke'
```

## Expected behavior
I'm expecting no error to be thrown.

Thanks for your interest! LUKE is not part of the master branch yet.

Actually, the current implementation of LUKE is here (at my `adding_luke_v2` branch): https://github.com/NielsRogge/transformers/tree/adding_luke_v2/src/transformers/models/luke

Note that it is work-in-progress, but you can already use the base `EntityAwareAttentionModel` and the head models. It's mostly the tokenizer that needs some work.

cc'ing the original author for visibility: @ikuyamada <|||||>Thanks, Niels!

As far as I'm concerned, this can be closed.