Fix HumanMessagePromptTemplate bug (#763)

HumanMessagePromptTemplate and some other MessagePromptTemplates were
not built correctly.

This fixes SystemMessage Example.

src/backend/langflow/graph/vertex/types.py

@@ -226,7 +226,12 @@ def _built_object_repr(self):
         # so the prompt format doesn't break
         artifacts.pop("handle_keys", None)
         try:
-            template = self._built_object.template
+            if not hasattr(self._built_object, "template") and hasattr(
+                self._built_object, "prompt"
+            ):
+                template = self._built_object.prompt.template
+            else:
+                template = self._built_object.template
             for key, value in artifacts.items():
                 if value:
                     replace_key = "{" + key + "}"

src/backend/langflow/interface/initialize/utils.py

@@ -51,7 +51,9 @@ def handle_partial_variables(prompt, format_kwargs: Dict):
     }
     # Remove handle_keys otherwise LangChain raises an error
     partial_variables.pop("handle_keys", None)
-    return prompt.partial(**partial_variables)
+    if partial_variables and hasattr(prompt, "partial"):
+        return prompt.partial(**partial_variables)
+    return prompt
 
 
 def handle_variable(params: Dict, input_variable: str, format_kwargs: Dict):