Add from_file method to message prompt template (#4713)

**Feature**: This PR adds `from_template_file` class method to
BaseStringMessagePromptTemplate. This is useful to help user to create
message prompt templates directly from template files, including
`ChatMessagePromptTemplate`, `HumanMessagePromptTemplate`,
`AIMessagePromptTemplate` & `SystemMessagePromptTemplate`.

**Tests**: Unit tests have been added in this PR.

Co-authored-by: charosen <charosen@bupt.cn>
Co-authored-by: Dev 2049 <dev.dev2049@gmail.com>

langchain/prompts/chat.py

@@ -74,6 +74,16 @@ class BaseStringMessagePromptTemplate(BaseMessagePromptTemplate, ABC):
         prompt = PromptTemplate.from_template(template)
         return cls(prompt=prompt, **kwargs)
 
+    @classmethod
+    def from_template_file(
+        cls: Type[MessagePromptTemplateT],
+        template_file: Union[str, Path],
+        input_variables: List[str],
+        **kwargs: Any,
+    ) -> MessagePromptTemplateT:
+        prompt = PromptTemplate.from_file(template_file, input_variables)
+        return cls(prompt=prompt, **kwargs)
+
     @abstractmethod
     def format(self, **kwargs: Any) -> BaseMessage:
         """To a BaseMessage."""

tests/unit_tests/prompts/test_chat.py

@@ -1,3 +1,4 @@
+from pathlib import Path
 from typing import List
 
 from langchain.prompts import PromptTemplate
@@ -80,6 +81,21 @@ def test_create_chat_prompt_template_from_template_partial() -> None:
     assert output_prompt.prompt == expected_prompt
 
 
+def test_message_prompt_template_from_template_file() -> None:
+    expected = ChatMessagePromptTemplate(
+        prompt=PromptTemplate(
+            template="Question: {question}\nAnswer:", input_variables=["question"]
+        ),
+        role="human",
+    )
+    actual = ChatMessagePromptTemplate.from_template_file(
+        Path(__file__).parent.parent / "data" / "prompt_file.txt",
+        ["question"],
+        role="human",
+    )
+    assert expected == actual
+
+
 def test_chat_prompt_template() -> None:
     """Test chat prompt template."""
     prompt_template = create_chat_prompt_template()