model : refactor QKV into common build_qkv and create_tensor_qkv helpers (#21245)

* model : refactor QKV into common build_qkv and create_tensor_qkv helpers

* model : extend build_qkv to bert/mpt/dbrx/olmo/lfm2/nemotron-h/granite-hybrid/gemma3n-iswa/t5-dec and fix wqkv_s

src/llama-graph.h

@@ -17,6 +17,7 @@ struct ggml_context;
 struct ggml_tensor;
 
 struct llama_cparams;
+struct llama_layer;
 
 struct llama_memory_context_i;
 
@@ -707,6 +708,12 @@ using llm_graph_result_ptr = std::unique_ptr<llm_graph_result>;
 // used in build_rs to properly order writes and avoid unnecessary copies
 using llm_graph_get_rows_fn = std::function<ggml_tensor * (ggml_context *, ggml_tensor * states, ggml_tensor * ids)>;
 
+struct llm_graph_qkv {
+    ggml_tensor * q; // [n_embd_head, n_head,    n_tokens]
+    ggml_tensor * k; // [n_embd_head, n_head_kv, n_tokens]
+    ggml_tensor * v; // [n_embd_head, n_head_kv, n_tokens]
+};
+
 struct llm_graph_context {
     const llm_arch arch;
 
@@ -793,6 +800,17 @@ struct llm_graph_context {
            llm_norm_type   type,
                      int   il) const;
 
+
+    // compute Q, K, V projections with optional bias and reshape
+    // supports both fused wqkv and separate wq/wk/wv paths
+    llm_graph_qkv build_qkv(
+        const llama_layer & layer,
+              ggml_tensor * cur,
+                  int64_t   n_embd_head,
+                  int64_t   n_head,
+                  int64_t   n_head_kv,
+                      int   il) const;
+
     ggml_tensor * build_ffn(
              ggml_tensor * cur,
              ggml_tensor * up,