Pixtral throws an error when using system role in chat.

[chigkim]

Llama vision works with --system message, but not Pixtral for some reason.

% mlx_vlm.generate --model mlx-community/pixtral-12b-8bit --image ../imgs/boat-256.png --system "You are a helpful assistant." --chat
Fetching 11 files: 100%|█████████████████████| 11/11 [00:00<00:00, 91907.06it/s]
User:Describe the image.
Traceback (most recent call last):
  File "/Users/cgk/Desktop/code/llm/mlx/.venv/bin/mlx_vlm.generate", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/cgk/Desktop/code/llm/mlx/mlx-vlm/mlx_vlm/generate.py", line 116, in main
    prompt = apply_chat_template(
             ^^^^^^^^^^^^^^^^^^^^
  File "/Users/cgk/Desktop/code/llm/mlx/mlx-vlm/mlx_vlm/prompt_utils.py", line 158, in apply_chat_template
    return processor.apply_chat_template(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cgk/Desktop/code/llm/mlx/.venv/lib/python3.12/site-packages/transformers/processing_utils.py", line 1131, in apply_chat_template
    return self.tokenizer.apply_chat_template(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cgk/Desktop/code/llm/mlx/.venv/lib/python3.12/site-packages/transformers/tokenization_utils_base.py", line 1683, in apply_chat_template
    rendered_chat = compiled_template.render(
                    ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/cgk/Desktop/code/llm/mlx/.venv/lib/python3.12/site-packages/jinja2/environment.py", line 1304, in render
    self.environment.handle_exception()
  File "/Users/cgk/Desktop/code/llm/mlx/.venv/lib/python3.12/site-packages/jinja2/environment.py", line 939, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "<template>", line 15, in top-level template code
TypeError: can only concatenate str (not "list") to str

[Blaizzy]

Perhaps we need to update the prompt utils.