examples: add keep_linebreaks option to CLM examples
Hi,

as discussed in #12971 a newline is missing when using the CLM example scripts, when no dataset name is provided (this is the case when you use "normal" text files).

This PR adds the `keep_linebreaks=True` option to all CLM example scripts (when using files).
Thanks a lot for your PR @stefan-it! 

Could we maybe make `keep_linebreaks` configurable by the command line and let it default to `True`? So ideally we could add it to the `DataArguments` class<|||||>@patrickvonplaten good idea, I'm working on it now!<|||||>I've implemented it :) 

The `examples/pytorch/language-modeling/run_clm_no_trainer.py` uses the raw argument parser, so I used the same boolean logic as used for the fast tokenizer (or here: slow tokenizer).<|||||>Thanks a lot for your PR @stefan-it! 

@sgugger agreed to make this change here: https://github.com/huggingface/transformers/issues/12971 

So good to merge for me!