Support MetaMath

[iojw]

Why are these changes needed?

• This adds support for the MetaMath models, including a new conversation template and model adapter.

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).

[BabyChouSr]

Thanks for your commit! Perhaps, to make the prompt template logic easier to follow, could you try using SeparatorStyle.NO_COLON_SINGLE with

system_prompt="Below is an instruction that describes a task. "
        "Write a response that appropriately completes the request.\n\n""
roles=("### Instruction:\n", "### Response: Let's think step by step.")
sep="\n\n"

Afterwards, test with the --debug flag with python3 -m fastchat.serve.cli command to see whether it is the same.

We try not to introduce too many new prompt templates if we can fit it into some previous template.

[iojw]

@BabyChouSr Thanks for the review! I think the issue with having the separators and message in the prompt is that scripts that rely on the roles will not display correctly. For example, using your suggestion, running python3 -m fastchat.serve.cli will print the following instead:

### Instruction:
: What is 2+2?
### Response: Let's think step by step.: First, we add the two numbers: 2 + 2 = 4.
Then, we add the result to 2: 4 + 2 = 6.
So, 2 + 2 = 6.
The answer is: 6

{'conv_template': 'metamath', 'prompt': "Below is an instruction that describes a task. Write a response that appropriately completes the request.\n\n### Instruction:\nWhat is 2+2?\n\n### Response: Let's think step by step.", 'outputs': 'First, we add the two numbers: 2 + 2 = 4.\nThen, we add the result to 2: 4 + 2 = 6.\nSo, 2 + 2 = 6.\nThe answer is: 6', 'speed (token/s)': 13.01}

The ultimate prompt is the same but the display is wrong, as compared to if we keep the roles as expected in the PR. What do you think?

[BabyChouSr]

I see how this can be an issue for existing functionality that depends on the roles being well-defined like our cli. In this case, I think you are right - we are willing to tradeoff the style for proper functionality in other components.

[infwinston]

@iojw sorry could you resolve conflicts? will merge this afterwards

[iojw]

@infwinston Fixed!