Import error in example script `run_language_modeling.py`
Author: xiaolul
Pull request: no

# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): RobertaForMaskedLM

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. pip install transformers
2. run `run_language_modeling.py`, which is the example script
3.

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Error message:
>  Traceback (most recent call last):                                                                                                                      File "run_language_modeling.py", line 42, in <module>                                                                                                   from transformers import ( 
ImportError: cannot import name 'MODEL_WITH_LM_HEAD_MAPPING'

## Expected behavior
The script should run..
<!-- A clear and concise description of what you would expect to happen. -->

## Environment info
<!-- You can run the command `python transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 2.5.1
- Platform: Ubuntu 18.04
- Python version: 3.7
- PyTorch version (GPU?): 1.4.0
- Tensorflow version (GPU?): na
- Using GPU in script?: y
- Using distributed or parallel set-up in script?: n


## Note
The work around is to use 
> from transformers.modeling_auto import MODEL_WITH_LM_HEAD_MAPPING
> from transformers.file_utils import WEIGHTS_NAME

Can you please update the example script? It is confusing ...



Comments:
You need to upgrade your version of transformers (to 2.6), or better, to [install from source](https://github.com/huggingface/transformers#run-the-examples).

I just pulled the `huggingface/transformers-tensorflow-gpu:2.10.0` docker image, went to the `examples/language-modeling/` folder and ran the following, and I got the same error:
```
python3 run_language_modeling.py --output_dir=/app/data --model_type=distilbert --model_name_or_path=distilbert-base-uncased --do_train --train_data_file=/app/data/train_data.txt  --do_eval --eval_data_file=/app/data/eval_data.txt  --mlm
```
Haven't tried the workaround above yet.


Steps:
- `docker run -it -v `pwd`/data:/app/data huggingface/transformers-tensorflow-gpu:2.10.0`
- `cd workspace/examples/language-modeling/`
- try to run example command using `python3`

`python3 -m pip show transformers` reports `2.10.0` is installed.


I get the issue (the `master` branch being checked out in the docker build) it just seems like it'd be cool for there to be a simpler way to run the examples in docker. If you wanted to use the `2.9.0` image, you'd have to pull the image and have your script first check out master as of the tag `2.9.0` then install from source, right?

It'd be a nice feature if the docker images could run the examples without modification

I get the same issue when I `pip install transformers`. When I downgrade to `2.6.0`, it can't import `CONFIG_MAPPING`. Anything from `2.7.0` to `2.10.0` up I get the `MODEL_WITH_LM_HEAD_MAPPING` error

Okay, I got it to work for `2.10.0`. I just had to reinstall PyTorch
```
pip3 install torch
```