[veomni] feat: support offloading/loading the veomni model/optimizer

[ji-huazhong]

What does this PR do?

This PR adds support for offloading both the model and optimizer (in veomni style) to CPU, as well as onloading them back to the device.

Additionally, it includes two model conversion scripts required by veomni:

• moe_merge.py: Converts models from Hugging Face (HF) format into a format compatible with veomni.
• moe_split.py: Converts checkpoints generated by veomni training back into HF format.

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[gemini-code-assist]

Code Review

This pull request introduces functionality to offload and load veomni models and optimizers to/from CPU, controlled by new configuration flags. The implementation is mostly sound, but I've identified a significant issue in the optimizer handling logic. The functions for offloading and loading optimizer state contain duplicated code and a potential bug that could prevent MultiOptimizer states from being processed correctly. I've provided suggestions to refactor this logic to improve its correctness and maintainability.