[cleanup] Unique constants in tests + env variable for inference tests

[Wauplin]

In this PR:

• Adds an autouse function-scoped fixture that overwrites the default HF_HOME with a temporary directory for each single test => all tests are made independent + it won't mess-up with local cache at all anymore. Let's check it doesn't break anything in CI.
• Sets the tokens for inference tests to HF_INFERENCE_TEST_TOKEN instead of HF_TOKEN. The problem of HF_TOKEN is that it is overwritten in pyproject.toml to "", making it ineffective. To update VCR tests locally, one has to manually edit pyproject.toml which is annoying.
  • => in practice, VCR tests seem to be skipped in CI because no token is set => this is independent from this PR and @hanouticelina is working on it (right?)
• Removes deprecated _old_token_path. Used only internally and deprecated since Nov. 2022 Store user token in huggingface home folder #1232

[hanouticelina]

in practice, VCR tests seem to be skipped in CI because no token is set => this is independent from this PR and @hanouticelina is working on it (right?)

yes, i will open the PR in a moment.
PS: I hate VCR 😄

[HuggingFaceDocBuilderDev]

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[hanouticelina]

looks good, thank you!

[hanouticelina]

in #2858, I removed completely API_KEY_ENV_VARIABLES to use HF_INFERENCE_TEST_TOKEN for all providers.
There are also other tests using HF_TOKEN, I updated them to use HF_INFERENCE_TEST_TOKEN instead.

[Wauplin]

Nice!