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-hparams.cpp

@@ -87,6 +87,15 @@ uint32_t llama_hparams::n_expert_used(uint32_t il) const {
     GGML_ABORT("fatal error");
 }
 
+uint32_t llama_hparams::n_expert_used_max() const {
+    uint32_t val = 0;
+    for (uint32_t il = 0; il < n_layer_all; ++il) {
+        val = std::max(val, n_expert_used(il));
+    }
+
+    return val;
+}
+
 uint32_t llama_hparams::n_gqa(uint32_t il) const {
     const uint32_t n_head    = this->n_head(il);
     const uint32_t n_head_kv = this->n_head_kv(il);