This has a basic example of using the Marvin library with OpenAI to classify, or using a grading rubric, on text. This doesn't pull the actual text from the LLM - the only API calls it makes are to OpenAI to do the grading.
Install the requirements.txt file and then explore the notebook Instructions_Example.
The tool requires an OpenAI API key for making calls to the language model. This key must be stored in the environment variable OPENAI_KEY.