Tensorflow Straggler Mitigation by Speculative Execution

[zhangpengshan]

Each iteration to do stats and check if any slow workers, check like STDDev and if any outlier worker could be run one as standby backup worker in backup pool.

[Mrhs121]

There is no need to do that. Backup has been implemented in TF. that means, each iteration only takes the fastest N workers and give up the slowest C Straggler.