Fix loading of ImagePromptTemplate (#16868)

We didn't override the namespace of the ImagePromptTemplate, so it is
listed as being in langchain.schema

This updates the mapping to let the loader deserialize.

Alternatively, we could make a slight breaking change and update the
namespace of the ImagePromptTemplate since we haven't broadly
publicized/documented it yet..

libs/core/langchain_core/load/mapping.py

@@ -115,6 +115,12 @@ SERIALIZABLE_MAPPING: Dict[Tuple[str, ...], Tuple[str, ...]] = {
         "chat",
         "SystemMessagePromptTemplate",
     ),
+    ("langchain", "prompts", "image", "ImagePromptTemplate"): (
+        "langchain_core",
+        "prompts",
+        "image",
+        "ImagePromptTemplate",
+    ),
     ("langchain", "schema", "agent", "AgentActionMessageLog"): (
         "langchain_core",
         "agents",
@@ -510,6 +516,12 @@ _OG_SERIALIZABLE_MAPPING: Dict[Tuple[str, ...], Tuple[str, ...]] = {
         "system",
         "SystemMessage",
     ),
+    ("langchain", "schema", "prompt_template", "ImagePromptTemplate"): (
+        "langchain_core",
+        "prompts",
+        "image",
+        "ImagePromptTemplate",
+    ),
 }
 
 # Needed for backwards compatibility for a few versions where we serialized

libs/core/langchain_core/prompts/image.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, List
 
 from langchain_core.prompt_values import ImagePromptValue, ImageURL, PromptValue
 from langchain_core.prompts.base import BasePromptTemplate
@@ -30,6 +30,11 @@ class ImagePromptTemplate(BasePromptTemplate[ImageURL]):
         """Return the prompt type key."""
         return "image-prompt"
 
+    @classmethod
+    def get_lc_namespace(cls) -> List[str]:
+        """Get the namespace of the langchain object."""
+        return ["langchain", "prompts", "image"]
+
     def format_prompt(self, **kwargs: Any) -> PromptValue:
         """Create Chat Messages."""
         return ImagePromptValue(image_url=self.format(**kwargs))