[feature] Fix template of Llama2 to match the offical template

src/llmtuner/extras/template.py

@@ -53,6 +53,22 @@ class Template:
         return convs
 
 
+class Llama2Template(Template):
+    def _format_example(self, query, history, prefix):
+        sys = prefix or self.prefix
+        if not sys.startswith("<<SYS>>\n"):
+            sys = f"<<SYS>>\n{sys.strip()}\n<</SYS>>\n\n"
+        history = history if (history and self.use_history) else []
+        history = history + [(query, "")]
+        convs = []
+        for turn_idx, (query_i, resp_i) in enumerate(history):
+            if turn_idx == 0:
+                convs.append([self.prompt.format(query=sys+query_i), resp_i])
+            else:
+                convs.append([(self.sep if turn_idx else prefix) + self.prompt.format(query=query_i), resp_i])
+        return convs
+
+
 templates: Dict[str, Template] = {}
 
 
@@ -101,8 +117,7 @@ Supports: https://huggingface.co/meta-llama/Llama-2-7b-chat-hf
           https://huggingface.co/meta-llama/Llama-2-13b-chat-hf
           https://huggingface.co/meta-llama/Llama-2-70b-chat-hf
 """
-register_template(
-    name="llama2",
+templates["llama2"] = Llama2Template(
     prefix="<<SYS>>\nYou are a helpful, respectful and honest assistant. "
            "Always answer as helpfully as possible, while being safe.  "
            "Your answers should not include any harmful, unethical, "
@@ -111,8 +126,8 @@ register_template(
            "If a question does not make any sense, or is not factually coherent, "
            "explain why instead of answering something not correct. "
            "If you don't know the answer to a question, please don't share false information.\n<</SYS>>\n\n",
-    prompt=" [INST] {query} [/INST] ",
-    sep="",
+    prompt="[INST]{query}[/INST]",
+    sep="<s>",
     use_history=True
 )