Hard to read predictions in JSQuAD

[kaisugi]

On the evaluation of JSQuAD, the prediction JSON file is output with unicodes escaped. This makes it difficult to check model outputs.

It would be better to add ensure_ascii=False in the following line

https://github.com/yahoojapan/JGLUE/blob/a6832af23895d6faec8ecf39ec925f1a91601d62/fine-tuning/patch/transformers-4.9.2_jglue-1.1.0.patch#L61

This change should be applied after merging #5, maybe?