run_mlm.py Issue | MODEL_FOR_MASKED_LM_MAPPING 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: 4.13.0.dev0
- Platform: Linux-3.10.0-1160.25.1.el7.x86_64-x86_64-with-centos-7.6.1810-Core
- Python version: 3.7.3
- PyTorch version (GPU?): not installed (NA)
- Tensorflow version (GPU?): 2.7.0 (True)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
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@sgugger @Rocketknight1 @Elysium1436 

## Information

Model I am using Bert:

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

The tasks I am working on is:
* [ language-modeling ] an official GLUE/SQUaD task: (give the name)

## To reproduce

Steps to reproduce the behavior:

1. prepare the env

```
python3 -m venv venv 

git clone https://github.com/huggingface/transformers
cd transformers
pip install .

pip install tensorflow
pip install datasets
pip install sklearn
```

2. run the script

```
python run_mlm.py \
--model_name_or_path distilbert-base-cased \
--output_dir output \
--dataset_name wikitext \
--dataset_config_name wikitext-103-raw-v1
```

3. get some error information

```
Traceback (most recent call last):
  File "run_mlm.py", line 63, in <module>
    MODEL_CONFIG_CLASSES = list(MODEL_FOR_MASKED_LM_MAPPING.keys())
AttributeError: 'NoneType' object has no attribute 'keys'
```


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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

I want to judge whether two lines of text should be merged into one line.
For example:

```
input: 

The preparations for the Beijing
Winter Olympics are progressing smoothly and are
fully recognized by the International Olympic
Committee, he said. 

output:

The preparations for the Beijing Winter Olympics are progressing smoothly and are fully recognized by the International Olympic Committee, he said.
```

I think maybe the masked language model can do this. I insert a `[MERGE]` or `[SPLIT]` special token into the gap of two lines and only masked these two tokens when construct masked tokens like this:

```
source input:
The preparations for the Beijing [MERGE] Winter Olympics are progressing smoothly and are [MERGE]  fully recognized by the International Olympic [MERGE] Committee, he said. 

masked input:
The preparations for the Beijing [mask] Winter Olympics are progressing smoothly and are [mask]  fully recognized by the International Olympic [mask] Committee, he said.
``` 

But when I try to execute the original script  `run_mlm.py` by Tutorials, I get the above error. What do I need to do to perform the training correctly? And do you think the task of merging sentences can be solved by language models?
I sovled the error by execute `pip install -r examples/pytorch/language-modeling/requirements.txt`, why should I install the requirements of pytorch example for tensorflow example?<|||||>Indeed the TensorFlow examples should use the TF mappings, cc @Rocketknight1 <|||||>Thank you for this bug report! We've added a PR to fix it, hopefully it will be merged soon.