Add integration test for checkpoint resharding

[shralex]

Adds a checkpoint resharding test - trains a model with a specific sharding strategy and saves a checkpoint.
Then train again by restoring this checkpoint using a different sharding strategy. Finally, asserts that the learning metrics are consistent, ensuring that checkpoints can be successfully loaded across different sharding strategies.

If the change fixes a bug or a Github issue, please include a link, e.g.,:
FIXES: b/504742746

Tests

This PR adds a test.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• 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.