You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: model.rs
    path of file: ./repos/llm-chain/crates/llm-chain-openai/src/chatgpt
    
    the code of the file until where you have to start completion: use llm_chain::options::{ModelRef, Opt};
use serde::{Deserialize, Serialize};
use strum_macros::EnumString;

/// The `Model` enum represents the available ChatGPT models that you can use through the OpenAI
/// API.
///
/// These models have different capabilities and performance characteristics, allowing you to choose
/// the one that best suits your needs. See <https://platform.openai.com/docs/models> for more
/// information.
///
/// # 
    
    fn test_to_string() {
        assert_eq!(Model::Gpt35Turbo.to_string(), "gpt-3.5-turbo");
        assert_eq!(Model::Gpt4.to_string(), "gpt-4");
        assert_eq!(Model::Gpt35Turbo0301.to_string(), "gpt-3.5-turbo-0301");
        assert_eq!(Model::Gpt4_0314.to_string(), "gpt-4-0314");
        assert_eq!(Model::Gpt4_32k.to_string(), "gpt-4-32k");
        assert_eq!(Model::Gpt4_32k0314.to_string(), "gpt-4-32k-0314");
        assert_eq!(
            Model::Other("custom_model".to_string()).to_string(),
            "custom_model"
        );
    }