model: try to improve Qwen3 Next (#18683)

* qwen3next: simplify qkvz projection

* use ggml_swiglu_split

* revert swiglu_split, but remove redundant repeat()

* fix missing reshape

* rm 2 redundant transposes

* move mul_mat(k,q) to outside of chunking

* rm redundant cont

* improve g_cs_chunk

* add comments about no cont

* use std::pair instead of ggml_concat

* vectorize key_gdiff calculation

* rm unused tensor

* avoid ggml_concat inside loop

* bring back ggml_concat as it may not work on other backend

* nits

gguf-py/gguf/constants.py

@@ -1738,6 +1738,7 @@ MODEL_TENSORS: dict[MODEL_ARCH, list[MODEL_TENSOR]] = {
         MODEL_TENSOR.ATTN_OUT,
         MODEL_TENSOR.ATTN_POST_NORM,
         MODEL_TENSOR.ATTN_GATE,
+        MODEL_TENSOR.ATTN_QKV,
         MODEL_TENSOR.FFN_GATE_INP,
         MODEL_TENSOR.FFN_GATE_INP_SHEXP,
         MODEL_TENSOR.FFN_UP_SHEXP,