Consistent type checks for prepend and append tags. (#1824)

Author: krammnic

torchtune/data/_prompt_templates.py

@@ -107,16 +107,17 @@ def __call__(
         """
         formatted_dialogue = []
         for message in messages:
+            content = message.content
             if message.role in self.template:
                 prepend_tag = self.template[message.role][0]
                 append_tag = self.template[message.role][1]
-                content = (
-                    [{"type": "text", "content": prepend_tag}]
-                    + message.content
-                    + [{"type": "text", "content": append_tag}]
-                )
-            else:
                 content = message.content
+
+                if isinstance(prepend_tag, str) and len(prepend_tag) > 0:
+                    content = [{"type": "text", "content": prepend_tag}] + content
+
+                if isinstance(append_tag, str) and len(append_tag) > 0:
+                    content = content + [{"type": "text", "content": append_tag}]
             formatted_dialogue.append(
                 Message(
                     role=message.role,
@@ -183,13 +184,20 @@ def __call__(
                 and index == len(messages) - 1
                 and len(message.text_content) == 0
             ):
-                content = [{"type": "text", "content": prepend_tag}] + message.content
+                content = message.content
+                if isinstance(prepend_tag, str) and len(prepend_tag) > 0:
+                    content = [
+                        {"type": "text", "content": prepend_tag}
+                    ] + message.content
             else:
-                content = (
-                    [{"type": "text", "content": prepend_tag}]
-                    + message.content
-                    + [{"type": "text", "content": append_tag}]
-                )
+                content = message.content
+
+                if isinstance(prepend_tag, str) and len(prepend_tag) > 0:
+                    content = [{"type": "text", "content": prepend_tag}] + content
+
+                if isinstance(append_tag, str) and len(append_tag) > 0:
+                    content = content + [{"type": "text", "content": append_tag}]
+
             formatted_dialogue.append(
                 Message(
                     role=message.role,