FIX: GPTCache cache_obj creation loop (#4827)

_get_gptcache method keep creating new gptcache instance, here's the fix

# Fix GPTCache cache_obj creation loop

Fixes #4830 

Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/cache.py

@@ -341,6 +341,8 @@ class GPTCache(BaseCache):
                 pre_embedding_func=get_prompt,
                 data_manager=get_data_manager(data_path=llm_string),
             )
+
+        self.gptcache_dict[llm_string] = _gptcache
         return _gptcache
 
     def _get_gptcache(self, llm_string: str) -> Any: