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/pai_eas_endpoint.py

@@ -6,8 +6,7 @@ import requests
 from langchain_core.callbacks import CallbackManagerForLLMRun
 from langchain_core.language_models.llms import LLM
 from langchain_core.outputs import GenerationChunk
-from langchain_core.pydantic_v1 import root_validator
-from langchain_core.utils import get_from_dict_or_env
+from langchain_core.utils import get_from_dict_or_env, pre_init
 
 from langchain_community.llms.utils import enforce_stop_tokens
 
@@ -52,7 +51,7 @@ class PaiEasEndpoint(LLM):
 
     version: Optional[str] = "2.0"
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key and python package exists in environment."""
         values["eas_service_url"] = get_from_dict_or_env(