Refactor: Improve Picture.py resource usage (#13011)

### What problem does this PR solve?

Improve Picture.py resource usage

### Type of change


- [x] Refactoring

rag/app/picture.py

@@ -78,7 +78,7 @@ 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)
-            img_binary = io.BytesIO()
+            with io.BytesIO() as img_binary:
                 img.save(img_binary, format="JPEG")
                 img_binary.seek(0)
                 ans = cv_mdl.describe(img_binary.read())