Add tests for RAG

[dcecchini]

We should add tests and benchmarks for RAG evaluation.

We can start with the ragas evaluation metric:

• Medium post
• Github repo

[ArshaanNazir]

Implement in library but give reference.

[RakshitKhajuria]

Evaluate LLMs and RAG a practical example using Langchain and Hugging Face

https://www.philschmid.de/evaluate-llm?ref=blog.langchain.dev