refactor(mimo): migrate MegatronMIMO to model builders

[yaoyu-33]

Summary

• migrate MegatronMIMO construction, conversion, checkpoint IO, and data loading to serializable ModelConfig plus standalone ModelBuilder contracts
• retain deprecated provider compatibility paths and released keyword aliases
• use one provider-neutral MegatronMIMOInfra type across legacy and builder paths
• pass runtime process grids explicitly from setup infrastructure instead of storing them in serializable configs
• update Qwen3.5-VL MIMO spec builders and repository examples

Stack

• Base: refactor(models): migrate AutoBridge to model builders #4545
• This PR contains only MegatronMIMO-specific migration changes and tests. Xiaomi MiMo model families remain in the base PR.

Validation

• uvx pre-commit run --all-files
• two independent subagent review passes completed; all confirmed findings fixed
• EOS: 628 targeted unit tests passed
• EOS: 2-GPU MegatronMIMO checkpoint save/load round-trip passed

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[yaoyu-33]

/ok to test 89f2971