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/gpt4all.py

@@ -3,7 +3,8 @@ from typing import Any, Dict, List, Mapping, Optional, Set
 
 from langchain_core.callbacks import CallbackManagerForLLMRun
 from langchain_core.language_models.llms import LLM
-from langchain_core.pydantic_v1 import Extra, Field, root_validator
+from langchain_core.pydantic_v1 import Extra, Field
+from langchain_core.utils import pre_init
 
 from langchain_community.llms.utils import enforce_stop_tokens
 
@@ -127,7 +128,7 @@ class GPT4All(LLM):
             "streaming": self.streaming,
         }
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that the python package exists in the environment."""
         try: