swap arg init in text_llm_runner (#14304)

Fix failures in [D82358803](https://www.internalfb.com/diff/D82358803)

(cherry picked from commit 0447ebd)

Author: lucylq

extension/llm/runner/text_llm_runner.cpp

@@ -43,7 +43,8 @@ TextLLMRunner::TextLLMRunner(
       io_manager_(std::move(io_manager)),
       text_token_generator_(std::move(text_token_generator)),
       stats_(std::move(stats)),
-      temperature_(temperature) {
+      temperature_(temperature),
+      pos_(0) {
   // Note: This constructor assumes that text_prefiller and text_token_generator
   // already have references to the Module and TextDecoderRunner they need
 }