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/llamacpp.py

@@ -8,7 +8,7 @@ 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 Field, root_validator
-from langchain_core.utils import get_pydantic_field_names
+from langchain_core.utils import get_pydantic_field_names, pre_init
 from langchain_core.utils.utils import build_extra_kwargs
 
 logger = logging.getLogger(__name__)
@@ -133,7 +133,7 @@ class LlamaCpp(LLM):
     verbose: bool = True
     """Print verbose output to stderr."""
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validate that llama-cpp-python library is installed."""
         try: