`GPTSimpleVectorIndex` is mentioned in [LlamaIndex setup](https://github.com/openai/chatgpt-retrieval-plugin/blob/main/docs/providers/llama/setup.md#setup) but the class is now renamed to `GPTVectorStoreIndex`: https://github.com/jerryjliu/llama_index/issues/1900