model : add Tencent Hy 4 (hy_v4) preview architecture support (#28127)

* model: add Tencent Hy 4 (hy_v4) preview architecture support

Adds support for the Tencent Hy 4 model (Hugging Face architecture
HYV4ForCausalLM, GGUF arch hy_v4):

Add HF -> GGUF conversion script (conversion/hy_v4.py) and wire it into the conversion registry
Register hy_v4 GGUF constants, arch enum, and writer support
Implement the hy-v4 model graph, hparams, vocab and context changes
Register the new arch in llama-arch and models registry
Extend arch tests to cover hy_v4

Assisted by Claude Opus 5

* Update convert_hf_to_gguf_update.py

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>

* Update conversion/base.py

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>

* convert : move hy_v4 entry to the same place as in convert_hf_to_gguf_update.py

* model : apply changes related to n_ff_exp becoming per-layer in Hy4-preview

* n_layer_all

---------

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>
Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>

conversion/__init__.py

@@ -124,6 +124,7 @@ TEXT_MODEL_MAP: dict[str, str] = {
     "HunYuanMoEV1ForCausalLM": "hunyuan",
     "HunYuanVLForConditionalGeneration": "hunyuan",
     "HYV3ForCausalLM": "hunyuan",
+    "HYV4ForCausalLM": "hy_v4",
     "IQuestCoderForCausalLM": "llama",
     "InternLM2ForCausalLM": "internlm",
     "InternLM3ForCausalLM": "internlm",