src : add n_expert_used_max function (#28323)

* src : add n_expert_used_max function

With Commit c61b98b875 ("model: add
NVIDIA Nemotron-3-Puzzle-75B-A9B (NemotronHPuzzle) support (#25444)") it
is now possible for each layer to have a specific number of experts but
there are a few checks that need to be updated to handle this upon model
loading. For example:
```console
llama_model_load: error loading model: model has expert layers but no expert layers are used
```
And later:
```console
/llama.cpp/src/llama-model-loader.cpp:955: GGML_ASSERT(n_ids_used > 0) failed
```

This commit adds the n_expert_used_max function so that these checks
can use it.

Refs: https://github.com/ggml-org/llama.cpp/pull/25444#issuecomment-5524976031

* src : use hparams.n_expert_used_max in llama_model_base::load_hparams

* src : use 0 as initial value for n_expert_used_max

src/llama-model.cpp

@@ -1254,10 +1254,7 @@ void llama_model_base::load_hparams(llama_model_loader & ml) {
     }
 
     // models may route a different number of experts per layer, so validate the maximum
-    uint32_t n_expert_used_max = 0;
-    for (uint32_t il = 0; il < hparams.n_layer_all; ++il) {
-        n_expert_used_max = std::max(n_expert_used_max, hparams.n_expert_used(il));
-    }
+    uint32_t n_expert_used_max = hparams.n_expert_used_max();
 
     GGML_ASSERT(hparams.n_expert <= LLAMA_MAX_EXPERTS);
     GGML_ASSERT(n_expert_used_max <= hparams.n_expert);
@@ -1509,10 +1506,9 @@ bool llama_model_base::load_tensors(llama_model_loader & ml) {
         // TODO: move to a separate function
         const auto tn = LLM_TN(arch);
 
-        const int64_t n_expert      = hparams.n_expert;
-        const int64_t n_expert_used = hparams.n_expert_used();
+        const int64_t n_expert = hparams.n_expert;
 
-        if (n_expert > 0 && n_expert_used == 0) {
+        if (n_expert > 0 && hparams.n_expert_used_max() == 0) {
             throw std::runtime_error("model has expert layers but no expert layers are used");
         }