model: MTP support for Qwen3-Next (#25589)

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 <redacted>
* rebase and add load_mtp flags

* Update src/models/qwen3next.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
* Update src/models/qwen3next.cpp

Co-authored-by: Sigbjørn Skjæret <redacted>
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Co-authored-by: Sigbjørn Skjæret <redacted>