langchain/chat_models/vertexai.py/ChatVertexAI/validate_environment
class ChatVertexAI:    def validate_environment(cls, values: Dict) -> Dict:
        """Validate that the python package exists in environment."""
        cls._try_init_vertexai(values)
        try:
            from vertexai.preview.language_models import ChatModel
        except ImportError:
            raise_vertex_import_error()
        values["client"] = ChatModel.from_pretrained(values["model_name"])
        return values