Optimize code (langchain-ai#3412)

Co-authored-by: assert <zhangchengming@kkguan.com>

Author: assert6

langchain/chains/combine_documents/stuff.py

@@ -40,8 +40,8 @@ class Config:
     @root_validator(pre=True)
     def get_default_document_variable_name(cls, values: Dict) -> Dict:
         """Get default document variable name, if not provided."""
+        llm_chain_variables = values["llm_chain"].prompt.input_variables
         if "document_variable_name" not in values:
-            llm_chain_variables = values["llm_chain"].prompt.input_variables
             if len(llm_chain_variables) == 1:
                 values["document_variable_name"] = llm_chain_variables[0]
             else:
@@ -50,7 +50,6 @@ def get_default_document_variable_name(cls, values: Dict) -> Dict:
                     "multiple llm_chain_variables"
                 )
         else:
-            llm_chain_variables = values["llm_chain"].prompt.input_variables
             if values["document_variable_name"] not in llm_chain_variables:
                 raise ValueError(
                     f"document_variable_name {values['document_variable_name']} was "