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
 }
 
 

rag/llm/chat_model.py

@@ -897,3 +897,9 @@ class OpenRouterChat(Base):
         self.base_url = "https://openrouter.ai/api/v1"
         self.client = OpenAI(base_url=self.base_url, api_key=key)
         self.model_name = model_name
+
+class StepFunChat(Base):
+    def __init__(self, key, model_name, base_url="https://api.stepfun.com/v1/chat/completions"):
+        if not base_url:
+            base_url = "https://api.stepfun.com/v1/chat/completions"
+        super().__init__(key, model_name, base_url)