[Executorch][sdpa] Setup the structure to enable quantized gemms for sdpa (#9912)

moving q_at_k and qk_at_v into separate functions to prepare for quantized sdpa

Differential Revision: [D71370607](https://our.internmc.facebook.com/intern/diff/D71370607/)

ghstack-source-id: 276012278
Pull Request resolved: #9889