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/watsonxllm.py

@@ -6,8 +6,8 @@ from langchain_core._api.deprecation import deprecated
 from langchain_core.callbacks import CallbackManagerForLLMRun
 from langchain_core.language_models.llms import BaseLLM
 from langchain_core.outputs import Generation, GenerationChunk, LLMResult
-from langchain_core.pydantic_v1 import Extra, SecretStr, root_validator
-from langchain_core.utils import convert_to_secret_str, get_from_dict_or_env
+from langchain_core.pydantic_v1 import Extra, SecretStr
+from langchain_core.utils import convert_to_secret_str, get_from_dict_or_env, pre_init
 
 logger = logging.getLogger(__name__)
 
@@ -115,7 +115,7 @@ class WatsonxLLM(BaseLLM):
             "instance_id": "WATSONX_INSTANCE_ID",
         }
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that credentials and python package exists in environment."""
         values["url"] = convert_to_secret_str(