Add autocheckpoint feature to MaxText.

[abhinavclemson]

Description

Add autocheckpoint feature to MaxText.

This change introduces enable_autocheckpoint, which allows Orbax to save a checkpoint when a preemption signal is detected. The checkpoint manager is configured with an AnySavePolicy that includes both preemption-based and fixed-interval saving.

Tests

Tested on tpu v7x with and without autocheckpoint enabled.

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[richjames0]

lgtm