Fit chinese wwm to new datasets
Sorry for my later update. 
I make my code(especially in chinese mlm_wwm) fit the newest code.
Here are the changes:
1. add `chinese_ref` key to avoid miss ref inf.
2. fix the type bug in `data_collator.py`
3. re-add `run_chinese_ref.py` cause it could run with the newest version code (4.2.2).
4. update readme
@sgugger  @LysandreJik 
Could you help me review these code ?<|||||>> Hi there! Thanks for updating your example. We have now created a `research_projects` project for the examples not directly maintained by the core team, and I think the `run_mlm_wwm` script and the chine_ref file could all go there in a new folder. Would you mind adjusting your PR in that direction?

Sure,  maybe move `run_chinese_ref.py` to  `research_projects` folder and leave `run_mlm_wwm.py` in where it was would be better ? And I don't know which folder is better ?
The two files are independent, we could move it to anywhere.<|||||>The `run_mlm_wwm` file is not maintained by us directly and it only works for BERT-models, compared to the other examples, so I think it can all go together there. You can create a new folder named `mlm_wwm` (since it's not just Chinese) for instance and have the specific requirements in the `requirements.txt` file there?<|||||>> The `run_mlm_wwm` file is not maintained by us directly and it only works for BERT-models, compared to the other examples, so I think it can all go together there. You can create a new folder named `mlm_wwm` (since it's not just Chinese) for instance and have the specific requirements in the `requirements.txt` file there?

done!<|||||>Last thing is to run `make style` to make sure the files are properly formatted, let me know if you have any issue doing this!<|||||>> Last thing is to run `make style` to make sure the files are properly formatted, let me know if you have any issue doing this!

yeah, seem my previous PR also failed in format :(
I got error as follow:
```
#!/bin/bash -eo pipefail
black --check examples tests src utils
would reformat /home/circleci/transformers/examples/research_projects/mlm_wwm/run_chinese_ref.py
would reformat /home/circleci/transformers/src/transformers/trainer.py
Oh no! 💥 💔 💥
2 files would be reformatted, 706 files would be left unchanged.

Exited with code exit status 1
```
But I formate my code.
![image](https://user-images.githubusercontent.com/16603773/106291319-713e9a80-6286-11eb-902b-a9240135ac15.png)

Maybe you could help me do this part ?<|||||>@sgugger My pleasure. Maybe you could help me fix the formate error :(
My python version `3.9.1` black `20.8b1`, why I got diff result in CI.