[Raiden Weight Sync 2/7] Support inhomogeneous layer cycle interval in raiden_unscan

[YixuanWang-99]

Overview

Part of the stacked Raiden weight-sync enablement PR chain replacing #5089.

Stack:

• [M7] [Raiden Weight Sync 1/7] Add cross-repo drift guard and FFI resolution tests #5166: Cross-repo drift guard
• [M1] AI-Hypercomputer/maxtext (this PR): Inhomogeneous layer cycle support in raiden_unscan
• [M2] AI-Hypercomputer/maxtext: MoE 128-lane layout and padding (pairs with Tunix T5)
• [M3] AI-Hypercomputer/maxtext: Target-free streaming weight conversion
• [M5] AI-Hypercomputer/maxtext: Shared expert gate weight_dtype fix
• [M4] AI-Hypercomputer/maxtext: TrainingEngine Raiden FFI integration (pairs with Tunix T2a)
• [M6] AI-Hypercomputer/maxtext: Rollout cleanup

Details

• Supports cycle_interval in unscan_layers for models with periodic inhomogeneous layers (e.g. Qwen3.5 GDN).
• Review thread addressed: When slot is None and cycle_interval > 1, raises a descriptive ValueError naming the key and the missing cycle-slot prefix instead of misleadingly attributing the failure to num_layers.
• Adds unit tests in tests/unit/raiden_unscan_test.py.

Verification

• pytest tests/unit/raiden_unscan_test.py (11/11 passed).

Checklist

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• 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.

[gemini-code-assist]

Code Review

This pull request introduces support for inhomogeneous layer cycles in unscan_layers by adding a cycle_interval parameter, which allows scanning blocks of heterogeneous layers and folding the slot index into the physical layer number. It also adds a comprehensive unit test suite for the unscan_layers functionality. The review feedback suggests optimizing the regular expression matching to avoid redundant calls, adding validation to ensure the parsed slot index is strictly less than the cycle interval to prevent silent weight overwriting, and expanding the unit tests to cover the new cycle_interval logic and its error paths.

[igorts-git]

please address review bot comments. I think they are meaningful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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