src : add n_expert_used_max function (#28323)
* src : add n_expert_used_max function
With Commit c61b98b875eaa5e654a3f5c73b34c310d2c6ab4c ("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
