refine TTS (#2500)

### What problem does this PR solve?

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/llm/tts_model.py

@@ -161,6 +161,7 @@ class QwenTTS(Base):
 
 class OpenAITTS(Base):
     def __init__(self, key, model_name="tts-1", base_url="https://api.openai.com/v1"):
+        if not base_url: base_url="https://api.openai.com/v1"
         self.api_key = key
         self.model_name = model_name
         self.base_url = base_url
@@ -181,6 +182,6 @@ class OpenAITTS(Base):
 
         if response.status_code != 200:
             raise Exception(f"**Error**: {response.status_code}, {response.text}")
-        for chunk in response.iter_content(chunk_size=1024):
+        for chunk in response.iter_content():
             if chunk:
                 yield chunk