train_file and validation_file in jsonl format with examples/pytorch/language-modeling/run_clm.py  
I'm using `./examples/pytorch/language-modeling/run_clm.py` providing the arguments _--train_file_ and _--validation_file_ with the **json** extension, however the underlying file is really a **jsonl** file.  This is confusing as I need to rename my jsonl files.

Since run_clm.py is compatible with jsonl files, is it possible to include the *jsonl* extension [here](https://github.com/huggingface/transformers/blob/eed3186b79d7ef9187113849f9ee7882bdd359fd/examples/pytorch/language-modeling/run_clm.py#L186) and [here](https://github.com/huggingface/transformers/blob/eed3186b79d7ef9187113849f9ee7882bdd359fd/examples/pytorch/language-modeling/run_clm.py#L189)?
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