add multi threaded mnist example
add multithreaded example


Former-commit-id: c934a079a21e31194802fefecb00719222791fae
add multithreaded example


Former-commit-id: 7f8750cd0989cc3d2ed34c4023d7e37eb06964ee
add multithreaded example


Former-commit-id: f7ad70023bdc2bd96984c79a61c9956c6191fbf1