model : fix qwen4exp layer inputs for DFlash drafters (#30927)

qwen4exp published the hc-wide residual [n_embd, hc, n_tokens] as
t_layer_inp, but the drafter reads layer inputs as [n_embd, n_tokens]
rows: llama_context sizes each embd_layer_inp buffer as n_embd * n_batch
and the DFlash encoder input is n_extract * n_embd wide. Evaluating a
DFlash draft over a qwen4exp target therefore aborts on the first request
in llama_context::extract_layer_inputs.

Collapse the streams with a plain mean before exposing them, as deepseek4
already does for its hyper-connection residual, and build the mean only
for the layers a drafter actually requested.

Assisted-by: pi:llama.cpp/ggml-org/Qwen3.8-Flash-Next-GGUF (MacBook, M5 Max, 128GB)

src/models/qwen4exp.cpp

@@ -308,6 +308,20 @@ std::unique_ptr<llm_graph_context> llama_model_qwen4exp::build_arch_graph(const
     return std::make_unique<graph>(*this, params);
 }
 
+// DFlash drafters read the layer inputs as [n_embd, T] rows, so collapse the streams
+static ggml_tensor * hc_stream_mean(ggml_context * ctx, ggml_tensor * x) {
+    const int64_t n_embd = x->ne[0];
+    const int64_t hc     = x->ne[1];
+    const int64_t nt     = x->ne[2];
+
+    ggml_tensor * acc = ggml_view_2d(ctx, x, n_embd, nt, x->nb[2], 0);
+    for (int64_t s = 1; s < hc; ++s) {
+        acc = ggml_add(ctx, acc, ggml_view_2d(ctx, x, n_embd, nt, x->nb[2], s*x->nb[1]));
+    }
+
+    return ggml_scale(ctx, acc, 1.0f / (float) hc);
+}
+
 // Hyper-connections keep hc parallel residual streams [n_embd, hc, T] in place of layer norms.
 // Returns the mixed [n_embd, T] stream; `inject` gets the [hc, T] scatter weights.
 ggml_tensor * llama_model_qwen4exp::graph::build_hc_mix(
@@ -447,7 +461,11 @@ llama_model_qwen4exp::graph::graph(const llama_model & model, const llm_graph_pa
     ggml_build_forward_expand(gf, res_hc);
 
     for (int il = 0; il < n_layer; ++il) {
-        res->t_layer_inp[il] = res_hc;
+        if ((size_t) il < cparams.embeddings_layer_inp.size() && cparams.embeddings_layer_inp[il]) {
+            res->t_layer_inp[il] = hc_stream_mean(ctx0, res_hc);
+            cb(res->t_layer_inp[il], "layer_inp", il);
+            ggml_build_forward_expand(gf, res->t_layer_inp[il]);
+        }
 
         if (hparams.is_ple(il)) {
             res_hc = build_ple(inp->get_recr(), ple_emb, res_hc, il);