dflash : clarify output logging of target_layer_ids (#27013)

This commit tries to make the logging of target_layer_ids a bit clearer
and easier to read.

Currently the output generated looks like this:
```console
0.00.468.624 D load_arch_hparams: DFlash extract_layers = [0.00.468.626 D 2, 0.00.468.626 D 6, 0.00.468.626 D 20,
  0.00.468.626 D 30, 0.00.468.627 D 42, 0.00.468.627 D 520.00.468.627 D ]
```
With the changes in the commit the output will be:
```console
0.00.522.765 D load_arch_hparams: DFlash extract_layers = [2, 6, 20, 30, 42, 52]
```

src/models/dflash.cpp

@@ -14,11 +14,14 @@ void llama_model_dflash::load_arch_hparams(llama_model_loader & ml) {
 
     hparams.n_embd_inp_enc_impl = (uint32_t) target_layer_ids.size() * hparams.n_embd;
 
-    LLAMA_LOG_INFO("%s: DFlash extract_layers = [", __func__);
+    std::string layers;
-    for (size_t i = 0; i < target_layer_ids.size(); ++i) {
+    const char * sep = "";
-        LLAMA_LOG_INFO("%d%s", target_layer_ids[i], i + 1 < target_layer_ids.size() ? ", " : "");
+    for (const auto id : target_layer_ids) {
+        layers += sep;
+        layers += std::to_string(id);
+        sep = ", ";
     }
-    LLAMA_LOG_INFO("]\n");
+    LLAMA_LOG_INFO("%s: DFlash extract_layers = [%s]\n", __func__, layers.c_str());
 
     // DeepSeek-V4 DSpark backbone: stages are full DSV4 blocks, uniform sliding window (the draft KV ring)
     ml.get_key(LLM_KV_HYPER_CONNECTION_COUNT, hparams.dsv4_hc_mult, false);