Better chat format for Qwen2.5-VL

[alcoftTAO]

CHAT_FORMAT = (
    "<|im_start|>system\n"
    "You are a helpful assistant.<|im_end|>\n"
    "{% for message in messages %}"
    "{% if message['role'] == 'user' %}"
    "<|im_start|>user\n"
    "{% if message['content'] is string %}"
    "{{ message['content'] }}"
    "{% else %}"
    "{% for content in message['content'] %}"
    "{% if content['type'] == 'text' %}"
    "{{ content['text'] }}"
    "{% elif content['type'] == 'image_url' %}"
    "{% if content.image_url is string %}"
    "{{ content.image_url }}"
    "{% else %}"
    "{{ content.image_url.url }}"
    "{% endif %}"
    "{% endif %}"
    "{% endfor %}"
    "{% endif %}"
    "<|im_end|>\n"
    "{% endif %}"
    "{% endfor %}"
    "<|im_start|>assistant\n"
)

This is the current chat format for Qwen2.5-VL, it works but sometimes the model prints a 'random' first token. Example: : In this image, I see...

This new chat format fixes it and also adds custom system messages for the model.