

TITLE
command line_by_line missing in https://github.com/huggingface/transformers/tree/master/examples/language-modeling

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## Environment info
- `transformers` version: 3.5.0
- Platform: macOS-10.16-x86_64-i386-64bit
- Python version: 3.8.6
- PyTorch version (GPU?): 1.7.0 (False)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: <fill in>
- Using distributed or parallel set-up in script?: <fill in>

### Who can help
@LysandreJik @patrickvonplaten @TevenLeScao


## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [ X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)
https://github.com/huggingface/transformers/tree/master/examples/language-modeling


## To reproduce

In the old version of the script ``run_clm.py`` called ``run_language_modeling`` there was an argument ``line_by_line`` which allowed to read the data by putting each sequence on a line. This argument seems to be missing in the newer version 
``run_clm.py``. 

## Expected behavior
Perhaps there is an argument that has replaced ``line_by_line``  but I don't really see that. Sorry if I missed something. 

