feat(backend): Support runtime weight refit

[TianyiZhao1437]

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• fix: Bug fix.
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💡 Description

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Key Changes

1.Support runtime weight downloading (echo -> lattica -> parallax) and saving to disk.
2.Only download weight that the node needs.
3.Support runtime weight update.

🔗 Related Issues

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• Closes #IssueNumber (e.g., Closes #123)

✅ Checklist

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• I have performed a self-review of my own code.
• I have added/updated tests that prove my fix or feature works (if applicable).
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