[llm] Route generate_from_pos() JNI API to native llm runner API (#11952)

Connect `generate_from_pos()` API to C++ llm runner's
`generate_from_pos()`.

Author: larryliu0820

extension/android/jni/jni_layer_llama.cpp

@@ -297,16 +297,27 @@ class ExecuTorchLlmJni : public facebook::jni::HybridClass<ExecuTorchLlmJni> {
       jlong start_pos,
       facebook::jni::alias_ref<ExecuTorchLlmCallbackJni> callback,
       jboolean echo) {
-    if (model_type_category_ != MODEL_TYPE_CATEGORY_MULTIMODAL) {
-      return static_cast<jint>(Error::NotSupported);
+    if (model_type_category_ == MODEL_TYPE_CATEGORY_MULTIMODAL) {
+      return static_cast<jint>(multi_modal_runner_->generate_from_pos(
+          prompt->toStdString(),
+          seq_len,
+          start_pos,
+          [callback](const std::string& result) { callback->onResult(result); },
+          [callback](const llm::Stats& stats) { callback->onStats(stats); },
+          echo));
+    } else if (model_type_category_ == MODEL_TYPE_CATEGORY_LLM) {
+      executorch::extension::llm::GenerationConfig config{
+          .echo = static_cast<bool>(echo),
+          .seq_len = seq_len,
+          .temperature = temperature_,
+      };
+      runner_->generate_from_pos(
+          prompt->toStdString(),
+          start_pos,
+          config,
+          [callback](std::string result) { callback->onResult(result); },
+          [callback](const llm::Stats& stats) { callback->onStats(stats); });
     }
-    return static_cast<jint>(multi_modal_runner_->generate_from_pos(
-        prompt->toStdString(),
-        seq_len,
-        start_pos,
-        [callback](const std::string& result) { callback->onResult(result); },
-        [callback](const llm::Stats& stats) { callback->onStats(stats); },
-        echo));
   }
 
   void stop() {