Support FAISS GPU

[m-hoseyny]

Hello,

Thanks for your project. I just want to add more GPU support for FAISS indexing and updating the readme to be more updated for future works.

Changes:

• Add GPU support in FAISS indexing
• Add an example for large query datasets
• Add an example to index MSMarco passages v1
• Fix minor problem in code (directory exists error)
• Add updated ANCE checkpoint download link

Thanks