llama : add llama_sampler_init for safe usage of llama_sampler_free (#11727)

The C API in llama.h claims users can implement `llama_sampler_i` to
create custom `llama_sampler`. The sampler chain takes ownership and
calls `llama_sampler_free` on them. However, `llama_sampler_free` is
hard-coded to use `delete`. This is undefined behavior if the object
wasn't also allocated via `new` from libllama's C++ runtime. Callers
in C and C-compatible languages do not use C++'s `new` operator. C++
callers may not be sharing the same heap as libllama.

common/llguidance.cpp

@@ -254,10 +254,10 @@ llama_sampler * llama_sampler_init_llg(const llama_vocab * vocab, const char * g
         };
     }
 
-    return new llama_sampler{
+    return llama_sampler_init(
         /* .iface = */ &llama_sampler_llg_i,
-        /* .ctx   = */ ctx,
-    };
+        /* .ctx   = */ ctx
+    );
 }
 
 #else