core[minor],community[minor]: Upgrade all @root_validator() to @pre_init (#23841)

This PR introduces a @pre_init decorator that's a @root_validator(pre=True) but with all the defaults populated!

libs/community/langchain_community/llms/openai.py

@@ -29,7 +29,11 @@ from langchain_core.callbacks import (
 from langchain_core.language_models.llms import BaseLLM, create_base_retry_decorator
 from langchain_core.outputs import Generation, GenerationChunk, LLMResult
 from langchain_core.pydantic_v1 import Field, root_validator
-from langchain_core.utils import get_from_dict_or_env, get_pydantic_field_names
+from langchain_core.utils import (
+    get_from_dict_or_env,
+    get_pydantic_field_names,
+    pre_init,
+)
 from langchain_core.utils.utils import build_extra_kwargs
 
 from langchain_community.utils.openai import is_openai_v1
@@ -269,7 +273,7 @@ class BaseOpenAI(BaseLLM):
         )
         return values
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         if values["n"] < 1:
@@ -818,7 +822,7 @@ class AzureOpenAI(BaseOpenAI):
         """Get the namespace of the langchain object."""
         return ["langchain", "llms", "openai"]
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         if values["n"] < 1:
@@ -1025,7 +1029,7 @@ class OpenAIChat(BaseLLM):
         values["model_kwargs"] = extra
         return values
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         openai_api_key = get_from_dict_or_env(