correct the llm type of AzureOpenAI (#3721)

The llm type of AzureOpenAI was previously set to default, which is
openai. But since AzureOpenAI has different API from openai, it creates
problems when doing chain saving and loading. This PR corrected the llm
type of AzureOpenAI to "azure"

langchain/llms/openai.py

@@ -589,6 +589,11 @@ class AzureOpenAI(BaseOpenAI):
     def _invocation_params(self) -> Dict[str, Any]:
         return {**{"engine": self.deployment_name}, **super()._invocation_params}
 
+    @property
+    def _llm_type(self) -> str:
+        """Return type of llm."""
+        return "azure"
+
 
 class OpenAIChat(BaseLLM):
     """Wrapper around OpenAI Chat large language models.