chain.stream should return None instead of Error when stream ends

[prabirshrestha]

Currently I need to write a lot of boilerplate code when the streaming has ended. this also means I need to add async_openai as dependeny.

let mut stream = chain.stream(input_variables).await?;

while let Some(result) = stream.next().await {
    match &result {
        Ok(data) => data.to_stdout()?,
        Err(ChainError::LLMError(LLMError::OpenAIError(
            OpenAIError::StreamError(e),
        ))) => {
            if e == "Stream ended" {
                break;
            } else {
                error!("OpenAI Error: {:?}", e);
                break;
            }
        }
        Err(e) => panic!("Errorx: {:?}", e),
    }
}

This could be simplified to use Result<Option<StreamData>>.

let mut stream = chain.stream(input_variables).await?;

while let Some(result) = stream.next().await {
    match &result {
        Ok(Some(data)) => data.to_stdout()?,
        None => break,
        Err(e) => panic!("Errorx: {:?}", e),
    }
}