llama : load MTP tensors only if they are really used (#26296)

* llama : load MTP tensors only if they are really used

* llama : skip loading MTP (if not used) in remaining models that support MTP

---------

Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>

include/llama.h

@@ -337,6 +337,7 @@ extern "C" {
         bool use_extra_bufts; // use extra buffer types (used for weight repacking)
         bool no_host;         // bypass host buffer allowing extra buffers to be used
         bool no_alloc;        // only load metadata and simulate memory allocations
+        bool load_mtp;        // whether to load MTP layers
     };
 
     struct llama_sampler_seq_config {