Function create_completion 

pub fn create_completion<C, Fut, B, E>(
    client: C,
    request: &CreateCompletionRequest,
) -> CreateCompletion<Fut, B, E> 
where
    C: FnOnce(Request<String>) -> Fut,
    Fut: Future<Output = Result<Response<B>, E>>,
    B: Body,

Create completion