Qwen2/3 MoE + GGUF model support (restored)
Author: a4lg
Pull request: yes

# What does this PR do?

This commit restores Qwen2/3 MoE + GGUF support in Transformers v5.

In this version, handling of MoE tensors are significantly changed so that support for all MoE + GGUF models ... (okay, actually) Qwen2/3 MoE models in Transformers v4 is now broken in v5.

This commit now adopts new tensor handling, along with extended `TensorProcessor` with capabilities to handle not only tensor data but also tensor mappings.
In this process, Qwen2/3 MoE-specific hack is moved to `Qwen2MoeTensorProcessor`, making the main function to look more model-agnostic.

This is fully tested on Qwen2 MoE `Qwen1.5-MoE-A2.7B` (with 14.3B total parameters) and partially on Qwen3 MoE `Qwen3-30B-A3B-Thinking-2507` (due to memory constraints).

## Future Possibilities

Portions of this change is written to be model-agnostic and easily replaceable.
If we decide to add more GGUF support to MoE models, we'd better to have either a mix-in or an utility.  In this case, a part of `Qwen2MoeTensorProcessor` can be copied to that with small modification.

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## Who can review?

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@CyrilVallez @SunMarc @MekkCyber

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Comments:
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