Something wrong with llama2 chat template?

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Hi,
Using the template in this repo gives MT-bench score 6.86 for Llama2-70B-chat, but using [llama repo](https://github.com/facebookresearch/llama) gives 7.11. The following template doesn't seem exactly the same as [this](https://github.com/facebookresearch/llama/blob/cfc3fc8c1968d390eb830e65c63865e980873a06/llama/generation.py#L212).
```
elif self.sep_style == SeparatorStyle.LLAMA2:
seps = [self.sep, self.sep2]
if self.system_message:
ret = system_prompt
else:
ret = "[INST] "
for i, (role, message) in enumerate(self.messages):
tag = self.roles[i % 2]
if message:
if i == 0:
ret += message + " "
else:
ret += tag + " " + message + seps[i % 2]
else:
ret += tag
return ret
```

Start by comparing the LLAMA2 template branch shown in the issue with llama/generation.py at the linked commit, then reproduce the two rendered prompts and their MT-bench results. Done means identifying whether the templates differ and confirming the corrected behavior with the evaluation described in the issue.

Written by the indexing model from the issue text.