[Executorch][llm] Add support for ring kv cache and ring attention (#10832)

Pull Request resolved: #10608

Introduced CachePositionManager to keep track of what is the position for each slot in ring kv cache. This is used to generate mask.
ghstack-source-id: 283404678
@exported-using-ghexport

Differential Revision: [D73891427](https://our.internmc.facebook.com/intern/diff/D73891427/)