speculative : fix prompt tokenization in speculative example (#4025)

* Support special tokens and not adding BOS to prompt in speculative

* Adapt to new should_add_bos function

* Ensure tgt and dft have same add_bos setting

examples/speculative/speculative.cpp

@@ -94,9 +94,22 @@ int main(int argc, char ** argv) {
         }
     }
 
-    // tokenize the prompt
+
+    // Tokenize the prompt
+    const bool add_bos_tgt = llama_should_add_bos_token(model_tgt);
+    LOG("add_bos tgt: %d\n", add_bos_tgt);
+
+    const bool add_bos_dft = llama_should_add_bos_token(model_dft);
+    LOG("add_bos dft: %d\n", add_bos_dft);
+
+    if (add_bos_tgt != add_bos_dft) {
+        fprintf(stderr, "%s: error: draft model add_bos must match target model to use speculation but ", __func__);
+        fprintf(stderr, "add_bos_dft = %d while add_bos_tgt = %d\n", add_bos_dft, add_bos_tgt);
+        return 1;
+    }
+
     std::vector<llama_token> inp;
-    inp = ::llama_tokenize(ctx_tgt, params.prompt, true);
+    inp = ::llama_tokenize(ctx_tgt, params.prompt, add_bos_tgt, true);
 
     const int max_context_size     = llama_n_ctx(ctx_tgt);
     const int max_tokens_list_size = max_context_size - 4;