[TENTATIVE] Updating variable names HUGGINGFACE_CO_RESOLVE_ENDPOINT into HF_ENDPOINT.
Author: Narsil
Pull request: yes

# What does this PR do?

`HUGGINGFACE_CO_RESOLVE_ENDPOINT` is deprecated and will go away in favor
of `HF_ENDPOINT`.

In order to maximize consistency, this PR renames the variable name itself.

This is tentative, since there might have been reasons to keep the old name for
the variable.

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@LysandreJik @sgugger

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Models:

- albert, bert, xlm: @LysandreJik
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Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

> This is purely cosmetic and internal, so I don't really see the point. We usually don't bother with renaming in those situations.

Is there any downsides to making the modifications ?

Consistency in naming has a lot of merits, especially within HF ecosystem where the name is used in many different places and different repos. Here for instance, it took me several tries to find all variables that needed to be overridden in `datasets`, `transformers` and `huggingface_hub`. Changing the name increases the chances that new code respects that convention.

I think the renaming here makes sense as that's the variable name in other repositories; since there are no downsides, ok for me!

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.

Ok, merging, feel free to revert if anything  breaks !