fix Llama-template's system prompt bug

[YeQiuO]

What does this PR do?

Llama 模型对应的 jinjia 模板有误，会导致系统提示被覆盖。

目前 Llama 的 jinjia 模板：

{% set system_message = '[template.default_system]' %}

{% if messages[0]['role'] == 'system' %}
    {% set system_message = messages[0]['content'] %}
{% endif %}

{% for message in messages %}
    {% set content = message['content'] %}
    
    {% if loop.index0 == 0 and system_message is defined %}
        {% set content = '<<SYS>>\n' + system_message + '\n<</SYS>>\n\n' + message['content'] %}
    {% endif %}
    
    {% if message['role'] == 'user' %}
        {{ '[INST] ' + content + ' [/INST]' }}
    {% elif message['role'] == 'assistant' %}
        {{ content + '' }}
    {% endif %}
{% endfor %}

该方法只能满足默认的system_message，用户自定义的 system_message 会被后一轮 user 覆盖。

为解决这一问题，修改了判断条件，自定义系统提示词时，跳过第一轮。

Fixes #5123

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• Did you read the contributor guideline?
• Did you write any new necessary tests?

[hiyouga]

LGTM