add support for StepFun (#1611)

### What problem does this PR solve?

#1561 

### Type of change
- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>

rag/llm/__init__.py

@@ -71,6 +71,7 @@ ChatModel = {
     "Bedrock": BedrockChat,
     "Groq": GroqChat,
     'OpenRouter':OpenRouterChat,
+    "StepFun":StepFunChat
 }