Do we use JSON lines or JSON only for run_translation.py in PyTorch?
### Who can help

Models:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj

## Information

The run_translation.py file requires that we pass `train_file` and `validation_file` as **json** file only, see:

https://github.com/huggingface/transformers/blob/master/examples/pytorch/translation/run_translation.py#L219

However, the [README](https://github.com/huggingface/transformers/tree/master/examples/pytorch/translation) states that

> The task of translation supports only custom JSONLINES files, with each line being a dictionary with a key "translation" and its value another dictionary whose keys is the language pair

Also in the example just above this line, it references jsonlines:

```bash
python examples/pytorch/translation/run_translation.py \
    --model_name_or_path t5-small \
    ...
    --train_file path_to_jsonlines_file \
    --validation_file path_to_jsonlines_file \
    ...
```

Similarly, from the argument description:

https://github.com/huggingface/transformers/blob/master/examples/pytorch/translation/run_translation.py#L115

I have made a merge request to address, what I believe, is an issue. Please correct me if I'm wrong. Thanks.