langchain/llms/vertexai.py/VertexAI/validate_environment
class VertexAI:    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 TextGenerationModel
        except ImportError:
            raise_vertex_import_error()
        tuned_model_name = values.get("tuned_model_name")
        if tuned_model_name:
            values["client"] = TextGenerationModel.get_tuned_model(tuned_model_name)
        else:
            values["client"] = TextGenerationModel.from_pretrained(values["model_name"])
        return values