Use store/restore for data sharing between gpus

[byeongjee]

depends on #4

---

before

average 0.0592s
total   1.1845s
end-to-end 1.1853s

after

average 0.0580s
total   1.1593s
end-to-end 1.1600s

TODO: do some refactoring and apply this to other benchmarks