add launch utility

[QiJune]

This PR depends on #375

First, install gotorch

go install ./...

Then, use launch tool to run 2 processes in a single node:

$GOPATH/bin/launch -nprocPerNode=2 -masterAddr=127.0.0.1 -masterPort=11111 -trainingCmd="$GOPATH/bin/allreduce"

It will run the allreduce example. Then, 0.log and 1.log will be created.

Check the 0.log:

cat 0.log
2020/11/02 17:54:47  2  4
 6  8
[ CPUFloatType{2,2} ]

You will find the value is allreduced correctly.