Support custom system message template

[zeyugao]

Why are these changes needed?

It makes the conversation class more adaptable, allowing for further customization of the chat.

conv = get_conv_template("llama-2")
conv.set_system_msg("You are a helpful, respectful and honest assistant.")
conv.append_message(conv.roles[0], "Hello!")
conv.append_message(conv.roles[1], "Hi!")
conv.append_message(conv.roles[0], "How are you?")
conv.append_message(conv.roles[1], None)
print(conv.get_prompt())

Results in

<s>[INST] <<SYS>>
You are a helpful, respectful and honest assistant.
<</SYS>>

Hello![/INST] Hi! </s><s>[INST] How are you? [/INST]

Related issue number (if applicable)

Checks

• I've run format.sh to lint the changes in this PR.
• I've included any doc changes needed.
• I've made sure the relevant tests are passing (if applicable).

[sarathkondeti]

conv.system is overwritten when inferencing from other modules/files.

For example in openai_api_server : https://github.com/lm-sys/FastChat/blob/4e2c942b8d785eb5e2aef1d0df2150e756f381ab/fastchat/serve/openai_api_server.py#L252C18-L252C18

Now, they need to use conv.system --> conv.system_msg.

[zeyugao]

@sarathkondeti That's true. I would try to fix it.

However, it appears there's an issue with the templates. I am unfamiliar with how each model is aligned with SFT. Hence, I'm uncertain whether the system prompt is fully predetermined, as with Stanford Alpaca, partially fixed, like Llama 2, or can be arbitrarily set, akin to one_shot and zero_shot during the fine-tuning process. This necessitates further investigation.

[laidybug]

@sarathkondeti That's true. I would try to fix it.

However, it appears there's an issue with the templates. I am unfamiliar with how each model is aligned with SFT. Hence, I'm uncertain whether the system prompt is fully predetermined, as with Stanford Alpaca, partially fixed, like Llama 2, or can be arbitrarily set, akin to one_shot and zero_shot during the fine-tuning process. This necessitates further investigation.

In my testing I've been able to arbitrarily change the system message and get useful results that are obviously influenced by it.

[zeyugao]

@merrymercy Sorry for that, it seems that I cannot allow the editing by maintainers due to the limitation from GitHub (https://github.com/orgs/community/discussions/5634) cause this pr is from a organisation instead of individual account. I didn't expect this problem.

It seems that you have created a new PR for that, so it is just fine to close this one for further modification.

[merrymercy]

@zeyugao Hi, thanks for the contributions!
It has been merged by #2135. You are added as a co-author.