hparams : refactor hparams.n_layer (#24060)

* hparams : refactor hparams.n_layer

* cont : remove `n_layer_kv()`, use n_layer_all instead

* cont : type consistency

* pi : update SYSTEM.md

* models : fix Step3.5 MTP

* cont : remove duplicate switch cases

* cont : explicitly set `false` to extra layers for `is_swa` and `is_recr`

* cont : fix nextn layer count handling

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

src/models/llada.cpp

@@ -2,14 +2,16 @@
 
 void llama_model_llada::load_arch_hparams(llama_model_loader & ml) {
     ml.get_key(LLM_KV_ATTENTION_LAYERNORM_RMS_EPS, hparams.f_norm_rms_eps);
+
     // LLaDA-8B has 32 layers, similar to LLaMA but for diffusion
-    switch (hparams.n_layer) {
+    switch (hparams.n_layer()) {
         case 32:
             type = LLM_TYPE_8B;
             break;
         default:
             type = LLM_TYPE_UNKNOWN;
     }
+
     // Set non-causal attention for diffusion models
     hparams.causal_attn = false;
 }