- Modified `chat_template` so that it supports the `system` role.

### Chat template

```jinja
{{ bos_token }}
{% if messages[0]['role'] == 'system' %}
    {% set loop_messages = messages[1:] %}
    {% set system_message = messages[0]['content'] + ' ' %}
{% else %}
    {% set loop_messages = messages %}
    {% set system_message = '' %}
{% endif %}
{% for message in loop_messages %}
    {% if (message['role'] == 'user') != (loop.index0 % 2 == 0) %}
        {{ raise_exception('Conversation roles must alternate user/assistant/user/assistant/...') }}
    {% endif %}
    {% if loop.index0 == 0 %}
        {% set content = system_message + message['content'] %}
    {% else %}
        {% set content = message['content'] %}
    {% endif %}
    {% if (message['role'] == 'assistant') %}
        {% set role = 'model' %}
    {% else %}
        {% set role = message['role'] %}
    {% endif %}
    {{ '<start_of_turn>' + role + '\n' + (content | trim) + '<end_of_turn>\n' }}
{% endfor %}
{% if add_generation_prompt %}
    {{'<start_of_turn>model\n'}}
{% endif %}
```