Fix text_llm_runner kv cache pos count and use it for generate() (pytorch#15286)

pos_ should advance by prefill and generated prompt size.

Author: kirklandsign

extension/llm/runner/text_llm_runner.cpp

@@ -190,7 +190,7 @@ Error TextLLMRunner::generate(
   // Generate max_new_tokens - 1 because prefill already generated 1 token.
   auto generate_result = text_token_generator_->generate(
       prompt_tokens,
-      num_prompt_tokens,
+      pos_,
       max_new_tokens - 1,
       temperature_ == -1.0f ? config.temperature : temperature_,
       wrapped_callback);
@@ -199,6 +199,8 @@ Error TextLLMRunner::generate(
   }
   int64_t num_generated_tokens = generate_result.get();
 
+  pos_ += num_generated_tokens;
+
   stats_->inference_end_ms = time_in_ms();
   if (!config.warming) {
     printf("\n");