Change JSON serialization to custom json.dumps
Author: junrae6454
Pull request: yes


# What does this PR do?
* This PR modifies the apply_chat_template function to improve handling of non-ASCII characters in JSON output by setting ensure_ascii to False. This change ensures that characters such as "안녕?" and emojis are correctly rendered in their original form rather than as escaped sequences.
* In the original tojson implementation of Jinja2, the characters "<", ">", "&", and "'" are escaped to ensure safe HTML output. However, in apply_chat_template, we use json.dumps directly to preserve the original content as much as possible.
[jinja2 default tojson](https://github.com/pallets/jinja/blob/8710cabd4fc58e38d1a83da3ed2d07e861d8bcdd/src/jinja2/utils.py#L657C12-L663)

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# relative issue
https://github.com/huggingface/transformers/pull/31041
https://github.com/huggingface/transformers/pull/31079

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Comments:
Yes, this makes sense, didn't catch the escaping of HTML characters.

May I suggest also removing the `sort_keys` parameter so it goes back to the default of not sorting keys, this way the caller has control over the order, which can improve inference.

@Rocketknight1 
When you have a moment, could you please review my latest pull request? Your feedback would be greatly appreciated.

Sorry for the delay! This makes sense to me, however there is a risk of affecting other Jinja implementations. @xenova how does `huggingface/jinja` deal with `|tojson`?

@Rocketknight1 AFAICT `huggingface.js/jinja` doesn't support `tojson` filter.

No, not yet, but it should be really easy to implement!

Okay! In that case, pinging @amyeroberts for core maintainer review.

Brief summary: We often want to flatten lists/dicts into a string in a chat template, especially when it deals with tools. Jinja was originally designed for templating in webpages, and as a result it escapes special HTML characters by default, but this is not what we want in a chat template. This PR swaps the default `tojson` filter in Jinja for Python's `json.dumps()` function, which doesn't have this awkward feature.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31100). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@junrae6454 Thanks for the PR! I made one small change - I moved the lambda function out into a proper function, and exposed a couple of the other arguments, as I feel like they might be useful later for rendering JSON.

Will merge once tests pass