Fix: change ollama default num_ctx. (#6395)

### What problem does this PR solve?

#6163

### Type of change

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

agent/component/categorize.py

         for c, desc in self.category_description.items():
             if desc.get("description"):
                 descriptions.append(
-                    "--------------------\nCategory: {}\nDescription: {}\n".format(c, desc["description"]))
+                    "\nCategory: {}\nDescription: {}".format(c, desc["description"]))
 
         self.prompt = """
-        You're a text classifier. You need to categorize the user’s questions into {} categories, 
-        namely: {}
-        Here's description of each category:
-        {}
-
-        You could learn from the following examples:
-        {}
-        You could learn from the above examples.
-        Just mention the category names, no need for any additional words.
-        
-        ---- Real Data ----
-        {}
+Role: You're a text classifier. 
+Task: You need to categorize the user’s questions into {} categories, namely: {}
+
+Here's description of each category:
+{}
+
+You could learn from the following examples:
+{}
+You could learn from the above examples.
+
+Requirements:
+- Just mention the category names, no need for any additional words.
+
+---- Real Data ----
+USER: {}\n
         """.format(
             len(self.category_description.keys()),
             "/".join(list(self.category_description.keys())),
             "\n".join(descriptions),
-            "- ".join(cate_lines),
+            "\n\n- ".join(cate_lines),
             chat_hist
         )
         return self.prompt

rag/llm/chat_model.py

         if "max_tokens" in gen_conf:
             del gen_conf["max_tokens"]
         try:
-            options = {}
+            options = {
+                "num_ctx": 32768
+            }
             if "temperature" in gen_conf:
                 options["temperature"] = gen_conf["temperature"]
             if "max_tokens" in gen_conf: