Fix jni_layer_llama.cpp generate_from_pos missing return (#12066)

Summary: Fix the issue on #11952 where we are missing a return in
generate_from_pos()

Reviewed By: GregoryComer

Differential Revision: D77456146

Author: larryliu0820

extension/android/jni/jni_layer_llama.cpp

@@ -311,12 +311,12 @@ class ExecuTorchLlmJni : public facebook::jni::HybridClass<ExecuTorchLlmJni> {
           .seq_len = seq_len,
           .temperature = temperature_,
       };
-      runner_->generate_from_pos(
+      return static_cast<jint>(runner_->generate_from_pos(
           prompt->toStdString(),
           start_pos,
           config,
           [callback](std::string result) { callback->onResult(result); },
-          [callback](const llm::Stats& stats) { callback->onStats(stats); });
+          [callback](const llm::Stats& stats) { callback->onStats(stats); }));
     }
   }