change back to json format after fixing eval() security issue

hf2llamastack.py

@@ -23,7 +23,7 @@ COT_PROMPT_TEMPATE = textwrap.dedent(
         ## Step 2: [Concise description]
         [Brief explanation]
         Regardless of the approach, always conclude with:
-        Answer :[the_answer_letter].
+        The best answer is [the_answer_letter].
         where the [the_answer_letter] is one of A, B, C or D.
         Let's think step by step.
         """
@@ -61,7 +61,7 @@ def convert_to_llamastack_format(ds):
 
         # must use the df.loc syntax, directly assigning to row['key'] won't keep it
         df.loc[idx, 'input_query'] = row['Question']
-        df.loc[idx, 'chat_completion_input'] = str(
+        df.loc[idx, 'chat_completion_input'] = json.dumps(
             [{
                 'role':'user', 'content':[{
                     'type':'text',