XLM tokenizer lang2id attribute is None
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.5.1
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.8.8
- PyTorch version (GPU?): 1.8.1+cpu (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: No

### Who can help

Models:

- albert, bert, xlm: @LysandreJik

Library:

- tokenizers: @LysandreJik

## Information

Model I am using XLM with Causal language modelling:

The problem arises when using:
* [x] the official example scripts: (give details below)

## To reproduce

Steps to reproduce the behaviour:

1. Run example code from https://huggingface.co/transformers/multilingual.html 

``` python
import torch
from transformers import XLMTokenizer, XLMWithLMHeadModel

tokenizer = XLMTokenizer.from_pretrained("xlm-clm-enfr-1024")
model = XLMWithLMHeadModel.from_pretrained("xlm-clm-enfr-1024")

language_id = tokenizer.lang2id['en']
```

The attribute lang2id is None and so I get a Nonetype is a non-suscriptable error. Following the example I am expecting to get 0 for language_id.

As a side note, it says that these checkpoints require language embeddings which I'm assuming is from the argument langs. What is the default behavior when this is not provided? I tried looking at https://huggingface.co/transformers/glossary.html but could not find any reference to it.

FYI, I tried downgrading and I found that the most recent version that doesn't have this bug is `transformers==4.3.3`. So you could try downgrading to that version for now, until someone fixes it.

```
pip install transformers==4.3.3
```<|||||>Thanks for the advice! So far I have just been manually specifying the language id for the two languages, hopefully, that is sufficient as well.<|||||>Hello! Sorry for taking so long to get back to this issue - the issue should normally be fixed now, for all versions. We updated the configurations of the XLM models on the hub.

Thanks for flagging!<|||||>Hi @LysandreJik is the update going to solve this XLM issue as well? https://github.com/huggingface/transformers/issues/12174<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.