Harmonize environment variables

[Wauplin]

From a discussion started by @thomwolf on slack (private link) cc @clefourrier @LysandreJik as well.

It's slightly error prone to have sometimes HF_ sometimes HUGGING_FACE_ and sometimes HUGGINGFACE_ in the env variables we use throuhout our libs (at least it tricked me a couple of times). Maybe some unification (with backward comp) to discuss across the libs as well at some point

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This PR fixes this by making all environments prefixed either by HF_ or HF_HUB_.
IMO, this will be particularly impactful for HF_TOKEN and HF_CACHE that were previously not intuitive.

4 environment variables have been modified:

Deprecated Variable	Replacement
HUGGINGFACE_HUB_CACHE	HF_HUB_CACHE
HUGGINGFACE_ASSETS_CACHE	HF_ASSETS_CACHE
HUGGING_FACE_HUB_TOKEN	HF_TOKEN
HUGGINGFACE_HUB_VERBOSITY	HF_HUB_VERBOSITY

Backward compatibility is of course kept. No warning triggered if a legacy variable is used. If both the legacy and replacement variables are set on the machine, the newest (replacement) variable is kept.

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Once this PR is merged and released, my plan will be to harmonize environment variables in the HF-ecosystem as well, starting with the Python libraries (transformers, datasets, diffusers, evaluate,...). But the goal is also to harmonize with JS, Rust and C# libraries when applicable.

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[stevhliu]

Love the little table, great job! 👍

[julien-c]

so it's harmonizing to transformers, right? (i.e. where do those HF_ env variables currently exist?)

[Wauplin]

It's about harmonizing between themselves. To avoid having HUGGINGFACE_HUB_CACHE, HUGGING_FACE_HUB_TOKEN and HF_HOME with three ways to spell huggingface. It is annoying as you always need to go back to the docs to check which one is the good one.

[julien-c]

HF_CACHE => i would rename this one to HF_HUB_CACHE (and nest HF_HUB_ASSETS_CACHE if it's under hub/)

Other than that lgtm

[Wauplin]

I'm fine with HF_HUB_CACHE instead of HF_CACHE. Made the change in 71b8880.

[LysandreJik]

Great! Thanks for doing the work here @Wauplin

Looks good to me. Let's make sure we update in transformers and others

[Wauplin]

Thanks for the review! I'll merge now.

Let's make sure we update in transformers and others

Yes definitely! I'll take care of it once released.