FIX: Automatic checkpoint path inference issue

[BenjaminBossan]

What does this PR do?

Resolves #1983

Note: I basically duplicated an existing test 99% to check for this bug. My reasoning was that duplication is better for this test than abstracting it. LMK if you prefer to abstract it instead.

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[HuggingFaceDocBuilderDev]

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

[SunMarc]

LGTM ! I think that we can leave it as it is. If needeed, we can do the abstraction in another PR as there is also another test that is very similar test_can_resume_training_with_folder

[muellerzr]

Thanks! There's some good stuff in here I'd want to extract out into the test utils certainly (esp your context manager) but overall lg2m :)