[core] langauge model like (#15180)

libs/core/langchain_core/language_models/__init__.py

@@ -1,6 +1,7 @@
 from langchain_core.language_models.base import (
     BaseLanguageModel,
     LanguageModelInput,
+    LanguageModelLike,
     LanguageModelOutput,
     get_tokenizer,
 )
@@ -16,4 +17,5 @@ __all__ = [
     "LanguageModelInput",
     "get_tokenizer",
     "LanguageModelOutput",
+    "LanguageModelLike",
 ]

libs/core/langchain_core/language_models/base.py

@@ -17,7 +17,7 @@ from typing_extensions import TypeAlias
 
 from langchain_core.messages import AnyMessage, BaseMessage, get_buffer_string
 from langchain_core.prompt_values import PromptValue
-from langchain_core.runnables import RunnableSerializable
+from langchain_core.runnables import Runnable, RunnableSerializable
 from langchain_core.utils import get_pydantic_field_names
 
 if TYPE_CHECKING:
@@ -49,11 +49,13 @@ def _get_token_ids_default_method(text: str) -> List[int]:
 
 
 LanguageModelInput = Union[PromptValue, str, List[BaseMessage]]
-LanguageModelOutput = TypeVar("LanguageModelOutput")
+LanguageModelOutput = Union[BaseMessage, str]
+LanguageModelLike = Runnable[LanguageModelInput, LanguageModelOutput]
+LanguageModelOutputVar = TypeVar("LanguageModelOutputVar", BaseMessage, str)
 
 
 class BaseLanguageModel(
-    RunnableSerializable[LanguageModelInput, LanguageModelOutput], ABC
+    RunnableSerializable[LanguageModelInput, LanguageModelOutputVar], ABC
 ):
     """Abstract base class for interfacing with language models.
 

libs/core/tests/unit_tests/language_models/test_imports.py

@@ -9,6 +9,7 @@ EXPECTED_ALL = [
     "LanguageModelInput",
     "LanguageModelOutput",
     "get_tokenizer",
+    "LanguageModelLike",
 ]