Add weight_decay >= 0 validation to TrainingArguments
Author: kaivalya-cyber
Pull request: yes
Labels: name: Code agent slop

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# What does this PR do?

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Fixes # (issue)

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### CI recap

**Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=47191)
**Latest run:** [28996505816:2](https://github.com/huggingface/transformers/actions/runs/28996505816)
**Result:** `failure` | **Jobs:** 2 | **Tests:** 22 | **Failures:** 1 | **Duration:** 2m 25s

> **Code quality check failed**: test jobs were skipped. Fix the code quality issues and push again to run tests.

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