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/embeddings/vertexai.py

@@ -8,7 +8,7 @@ from typing import Any, Dict, List, Literal, Optional, Tuple
 from langchain_core._api.deprecation import deprecated
 from langchain_core.embeddings import Embeddings
 from langchain_core.language_models.llms import create_base_retry_decorator
-from langchain_core.pydantic_v1 import root_validator
+from langchain_core.utils import pre_init
 
 from langchain_community.llms.vertexai import _VertexAICommon
 from langchain_community.utilities.vertexai import raise_vertex_import_error
@@ -33,7 +33,7 @@ class VertexAIEmbeddings(_VertexAICommon, Embeddings):
     show_progress_bar: bool = False
     """Whether to show a tqdm progress bar. Must have `tqdm` installed."""
 
-    @root_validator()
+    @pre_init
     def validate_environment(cls, values: Dict) -> Dict:
         """Validates that the python package exists in environment."""
         cls._try_init_vertexai(values)