Fix RGBA error (#3707)

### What problem does this PR solve?

**Passing cv_mdl.describe() is not an RGB converted image**

### Type of change

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

rag/app/picture.py

@@ -41,7 +41,10 @@ def chunk(filename, binary, tenant_id, lang, callback=None, **kwargs):
     try:
         callback(0.4, "Use CV LLM to describe the picture.")
         cv_mdl = LLMBundle(tenant_id, LLMType.IMAGE2TEXT, lang=lang)
-        ans = cv_mdl.describe(binary)
+        img_binary = io.BytesIO()
+        img.save(img_binary, format='JPEG')
+        img_binary.seek(0)
+        ans = cv_mdl.describe(img_binary.read())
         callback(0.8, "CV LLM respond: %s ..." % ans[:32])
         txt += "\n" + ans
         tokenize(doc, txt, eng)