[bugfix] fix megatron model merger

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What does this PR do?

Fix megatron model merger.

High-Level Design

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Specific Changes

• Fix get rank method to support just TP.
• Fix state_dict keys after convert.
• Add mla/moe convert support.

API

Demonstrate how the API changes if any.

Usage Example

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Test

Test with Qwen3-8B and Qwen2.5-7B.

Additional Info.

• Issue Number: Fixes issue #1757
• Training: [Note which backend this PR will affect: FSDP, Megatron, both, or none]
• Inference: [Note which backend this PR will affect: vLLM, SGLang, both, or none]

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