add support for transformers 5.2

[xadupre]

This PR ensures transformers works with transformers>=5.2. However some tests are disabled because optimum and diffusers do not support this version yet (import issues for example).

[HuggingFaceDocBuilderDev]

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[lagranhe]

@xadupre could you please take a look on blocking issues of this PR? It will be great to have it merged

[stharrold]

Watching this from a downstream user — thanks for the work, and for transparently calling out the optimum + diffusers compatibility issues blocking the merge.

Two questions in case they're useful:

1. Anything contributors can help with on the optimum / diffusers side? If the import-issue blockers are tracked elsewhere (separate issue / PR / discussion), a link from this PR's body would help downstream watchers know what to nudge or test.
2. Once transformers 5.2 lands, is the plan to bump iteratively to 5.7+ in follow-up PRs, or merge a single later-version bump? Asking because Gemma-4 specifically needs transformers>=5.7, which we've cited on transformers>=5.0.0 support #114 with the production traceback. Just want to set realistic expectations on whether this PR is the unblock for Gemma-4 support or a stepping stone.

Happy to test a pre-release build against a Gemma-4-E2B merged-LoRA checkpoint when ready. No pressure — appreciate the work in flight.