model: MTP support for Qwen3-Next (#25589)
* mtp for qwen3nex
* fix for python type-check
* Fix to compute num_mtp from directly mtp layer
* define opt_num_mtp_layers in _QwenMtpMixin and fix some comments
* Fix for python type check
* Update gguf-py/gguf/constants.py
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>
* rebase and add load_mtp flags
* Update src/models/qwen3next.cpp
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>
* Update src/models/qwen3next.cpp
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>
