Add support for Gemma chat template (#1530)

* Add support for Gemma chat template

* Update fschat version to include its newest support for Gemma chat style

* pin fastchat to current HEAD

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Co-authored-by: Wing Lian <wing.lian@gmail.com>

src/axolotl/monkeypatch/fastchat_conversation_turns.py

@@ -123,6 +123,14 @@ def get_turns(  # pylint: disable=too-many-return-statements
             else:
                 yield role, ""
         return
+    if self.sep_style == SeparatorStyle.GEMMA:
+        if self.system_message:
+            raise ValueError("Gemma chat template does not support system messages")
+        for i, (role, message) in enumerate(self.messages):
+            prefix = "<bos>" if i == 0 else ""
+            message_str = message if message else ""
+            yield prefix + "<start_of_turn>" + role + "\n", message_str + "<end_of_turn>\n"
+        return
     if self.sep_style == SeparatorStyle.CHATGLM:
         # source: https://huggingface.co/THUDM/chatglm-6b/blob/1d240ba371910e9282298d4592532d7f0f3e9f3e/modeling_chatglm.py#L1302-L1308
         # source2: https://huggingface.co/THUDM/chatglm2-6b/blob/e186c891cf64310ac66ef10a87e6635fa6c2a579/modeling_chatglm.py#L926