Gemma 2: Gemma template won't end with eos token

Nobody has claimed this yet.

For the current gemma template, the template is such that the marks the end of each turn and the last token in the sequence. However, for [Gemma 2 (Table 4)](https://arxiv.org/pdf/2408.00118), the last token should be the eos token. For a workaround, I modified [conversation.py](https://github.com/lm-sys/FastChat/blob/587d5cfa1609a43d192cedb8441cac3c17db105d/fastchat/conversation.py) like so

```
if self.sep_style == SeparatorStyle.GEMMA:
ret = ""
for role, message in self.messages:
if message:
ret += "" + role + "\n" + message + self.sep
else:
ret += "" + role + "\n"
if self.stop_str == "":
ret += ""
```

and registering it like so:

```
register_conv_template(
Conversation(
name="gemma-2",
roles=("user", "model"),
sep_style=SeparatorStyle.GEMMA,
sep="\n",
stop_str="",
),
)
```

I hope this helps other people that run into this!

Start in fastchat/conversation.py and inspect the current SeparatorStyle.GEMMA rendering and template registration. Compare the rendered Gemma 2 sequence with Table 4 of the linked paper, then verify that the final token is the eos token rather than only the end-of-turn marker.

Written by the indexing model from the issue text.