core[patch]: improve PydanticOutputParser typing (#18740)

This PR adds generic typing to `PydanticOutputParser` so we get a typed
output from `.parse` instead of `Any`. It should provide a better DX by
way of Intellisense and for anyone strictly typing.

Pre-change:

![Screenshot 2024-03-07 at 10 22
31 AM](https://github.com/langchain-ai/langchain/assets/22690160/fd22dde0-9fdc-4283-b283-4c98f0bc46e5)

Post-change:

![Screenshot 2024-03-07 at 10 26
31 AM](https://github.com/langchain-ai/langchain/assets/22690160/7e23d2b7-8f8c-494f-80b3-187530a173ee)

I haven't dug too deep, but I think a similar change could probably be
added to `JsonOutputParser` so we don't have to pull up `.parse`.

Co-authored-by: Jan Nissen <jan23@gmail.com>

libs/core/langchain_core/output_parsers/pydantic.py

@@ -1,23 +1,27 @@
 import json
-from typing import Any, List, Type
+from typing import Generic, List, Type, TypeVar
 
 from langchain_core.exceptions import OutputParserException
 from langchain_core.output_parsers import JsonOutputParser
 from langchain_core.outputs import Generation
 from langchain_core.pydantic_v1 import BaseModel, ValidationError
 
+TBaseModel = TypeVar("TBaseModel", bound=BaseModel)
 
-class PydanticOutputParser(JsonOutputParser):
+
+class PydanticOutputParser(JsonOutputParser, Generic[TBaseModel]):
     """Parse an output using a pydantic model."""
 
-    pydantic_object: Type[BaseModel]
+    pydantic_object: Type[TBaseModel]
     """The pydantic model to parse.
     
     Attention: To avoid potential compatibility issues, it's recommended to use
         pydantic <2 or leverage the v1 namespace in pydantic >= 2.
     """
 
-    def parse_result(self, result: List[Generation], *, partial: bool = False) -> Any:
+    def parse_result(
+        self, result: List[Generation], *, partial: bool = False
+    ) -> TBaseModel:
         json_object = super().parse_result(result)
         try:
             return self.pydantic_object.parse_obj(json_object)
@@ -26,6 +30,9 @@ class PydanticOutputParser(JsonOutputParser):
             msg = f"Failed to parse {name} from completion {json_object}. Got: {e}"
             raise OutputParserException(msg, llm_output=json_object)
 
+    def parse(self, text: str) -> TBaseModel:
+        return super().parse(text)
+
     def get_format_instructions(self) -> str:
         # Copy schema to avoid altering original Pydantic schema.
         schema = {k: v for k, v in self.pydantic_object.schema().items()}
@@ -46,7 +53,7 @@ class PydanticOutputParser(JsonOutputParser):
         return "pydantic"
 
     @property
-    def OutputType(self) -> Type[BaseModel]:
+    def OutputType(self) -> Type[TBaseModel]:
         """Return the pydantic model."""
         return self.pydantic_object