split dequant functions from kv_cache.cu (#4787)

Summary:
Pull Request resolved: #4787

X-link: facebookresearch/FBGEMM#1811

Split the file up to get a bit more parallelism during build. I was finding it taking a long time to rebuild after I change one of the dequantization functions.

Reviewed By: cthi

Differential Revision: D81135633

fbshipit-source-id: 804950ea42c244bf3c617370d27fc0d266fc3e89

Author: bottler