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/embeddings/localai.py

@@ -17,7 +17,11 @@ from typing import (
 
 from langchain_core.embeddings import Embeddings
 from langchain_core.pydantic_v1 import BaseModel, Extra, 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 tenacity import (
     AsyncRetrying,
     before_sleep_log,
@@ -193,7 +197,7 @@ class LocalAIEmbeddings(BaseModel, Embeddings):
         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."""
         values["openai_api_key"] = get_from_dict_or_env(