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/fireworks.py

@@ -9,8 +9,8 @@ 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, SecretStr, root_validator
-from langchain_core.utils import convert_to_secret_str
+from langchain_core.pydantic_v1 import Field, SecretStr
+from langchain_core.utils import convert_to_secret_str, pre_init
 from langchain_core.utils.env import get_from_dict_or_env
 
 
@@ -61,7 +61,7 @@ class Fireworks(BaseLLM):
         """Get the namespace of the langchain object."""
         return ["langchain", "llms", "fireworks"]
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that api key in environment."""
         try: