Warn on server warning 'X-HF-Warning'

[Wauplin]

This PR adds a helper to emit warnings on server request. This can be useful when a request completed successfully but we still want to warn the user about something. In practice we are checking the X-HF-Warning response header when hf_raise_for_status is called.

Expected format:

'X-HF-Warning: topic; message'

To avoid spamming the user, only 1 warning per topic is printed to the user. The topic itself is not printed, it is just a way to deduplicate them. Topic deduplication is case-insensitive. Topic is optional, in which case only the first non-labelled warning is printed.

A single HTTP request can contain several warnings by setting several times the X-HF-Warning (TIL headers are not unique^^).

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Other alternatives explored:

• Warning header: long-deprecated and not used in practice
• Content-Warning draft: only a draft -so not yet a standard- + too much complexity for our use case. In this draft proposal, the warning message is set in the JSON body which is not as flexible as our approach. Warnings in body is theoretically more robust and powerful -with no size limit- but for our use case, using the header is fine (+ there is no body in HEAD calls + body is not always a JSON).

[HuggingFaceDocBuilderDev]

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[hanouticelina]

Looks good, thanks! I left one comment about whether we should show all topic-less warnings or not

[julien-c]

maybe use this PR to give a bit of context to the moon-landing team btw (in a moon-landing PR?) as i haven't yet shared a lot of context:)

[LysandreJik]

Looks good to me, nice and robust test

[Wauplin]

maybe use this PR to give a bit of context to the moon-landing team btw (in a moon-landing PR?) as i haven't yet shared a lot of context:)

I've created an issue -internal link- in moon with explanations (didn't want to start a PR just to give some details^^)

[severo]

A single HTTP request can contain several warnings by setting several times the X-HF-Warning (TIL headers are not unique^^).

thanks!