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-loader.cpp

@@ -951,7 +951,7 @@ static bool weight_buft_supported(const llama_hparams & hparams, ggml_tensor * w
         case GGML_OP_MUL_MAT_ID:
             {
                 // Used for either MoE expert routing or embedded adapter routing
-                const int n_ids_used = hparams.router_layer >= 0 ? 1 : hparams.n_expert_used();
+                const int n_ids_used = hparams.router_layer >= 0 ? 1 : hparams.n_expert_used_max();
                 GGML_ASSERT(n_ids_used > 0);
                 ggml_tensor * b = ggml_new_tensor_3d(ctx, GGML_TYPE_F32, w->ne[0], n_ids_used, 512);
                 ggml_tensor * ids = ggml_new_tensor_2d(ctx, GGML_TYPE_I32, n_ids_used, 512);
@@ -964,7 +964,7 @@ static bool weight_buft_supported(const llama_hparams & hparams, ggml_tensor * w
             } break;
         case GGML_OP_ADD_ID:
             {
-                const int n_expert_used = hparams.n_expert_used();
+                const int n_expert_used = hparams.n_expert_used_max();
                 GGML_ASSERT(n_expert_used > 0);
                 ggml_tensor * a = ggml_new_tensor_3d(ctx, GGML_TYPE_F32, w->ne[0], n_expert_used, 512);
                 ggml_tensor * c = ggml_new_tensor_2d(ctx, GGML_TYPE_I32, n_expert_used, 512);