[R] Add early stop callback function (#3938)
[R] Add early stop callback function (#3938)

Adding early stop round function
LinkHS/incubator-mxnet,DickJC123/mxnet,solin319/incubator-mxnet,madjam/mxnet,tlby/mxnet,Mega-DatA-Lab/mxnet,gautamkmr/incubator-mxnet,ForkedReposBak/mxnet,formath/mxnet,solin319/incubator-mxnet,luoyetx/mxnet,apache/incubator-mxnet,Ldpe2G/mxnet,yajiedesign/mxnet,ShownX/incubator-mxnet,Ldpe2G/mxnet,piiswrong/mxnet,Northrend/mxnet,navrasio/mxnet,ShownX/incubator-mxnet,weleen/mxnet,EvanzzzZ/mxnet,vikingMei/mxnet,hpi-xnor/BMXNet,DickJC123/mxnet,stefanhenneking/mxnet,sxjscience/mxnet,EvanzzzZ/mxnet,Ldpe2G/mxnet,jamesliu/mxnet,ShownX/incubator-mxnet,smolix/incubator-mxnet,hotpxl/mxnet,mbaijal/incubator-mxnet,ptrendx/mxnet,CodingCat/mxnet,piiswrong/mxnet,yajiedesign/mxnet,antoan2/incubator-mxnet,jiajiechen/mxnet,antoan2/incubator-mxnet,LinkHS/incubator-mxnet,vikingMei/mxnet,Northrend/mxnet,thirdwing/mxnet,rishita/mxnet,weleen/mxnet,apaleyes/mxnet,larroy/mxnet,saurabh3949/mxnet,dmlc/mxnet,leezu/mxnet,Guneet-Dhillon/mxnet,coder-james/mxnet,DR08/mxnet,apaleyes/mxnet,szha/mxnet,arank/mxnet,smolix/incubator-mxnet,leezu/mxnet,dmlc/mxnet,navrasio/mxnet,Guneet-Dhillon/mxnet,ptrendx/mxnet,rishita/mxnet,dmlc/mxnet,xcgoner/dist-mxnet,sxjscience/mxnet,luoyetx/mxnet,kevinthesun/mxnet,coder-james/mxnet,LinkHS/incubator-mxnet,DR08/mxnet,szha/mxnet,ForkedReposBak/mxnet,luoyetx/mxnet,zhreshold/mxnet,jamesliu/mxnet,kkk669/mxnet,larroy/mxnet,arank/mxnet,zhreshold/mxnet,coder-james/mxnet,hpi-xnor/BMXNet,LinkHS/incubator-mxnet,yuruofeifei/mxnet,antoan2/incubator-mxnet,gautamkmr/incubator-mxnet,fullfanta/mxnet,DR08/mxnet,vikingMei/mxnet,piiswrong/mxnet,danithaca/mxnet,eric-haibin-lin/mxnet,fullfanta/mxnet,vikingMei/mxnet,Northrend/mxnet,sergeykolychev/mxnet,jermainewang/mxnet,Guneet-Dhillon/mxnet,saurabh3949/mxnet,tornadomeet/mxnet,jennyzhang0215/incubator-mxnet,formath/mxnet,hotpxl/mxnet,xcgoner/dist-mxnet,hesseltuinhof/mxnet,eric-haibin-lin/mxnet,hotpxl/mxnet,indhub/mxnet,dmlc/mxnet,jiajiechen/mxnet,zhreshold/mxnet,madjam/mxnet,piiswrong/mxnet,weleen/mxnet,sxjscience/mxnet,jiajiechen/mxnet,reminisce/mxnet,rahul003/mxnet,apache/incubator-mxnet,mbaijal/incubator-mxnet,Guneet-Dhillon/mxnet,ykim362/mxnet,piiswrong/mxnet,Northrend/mxnet,leezu/mxnet,dmlc/mxnet,apaleyes/mxnet,CodingCat/mxnet,madjam/mxnet,piiswrong/mxnet,DickJC123/mxnet,nicklhy/mxnet,kevinthesun/mxnet,ptrendx/mxnet,coder-james/mxnet,xcgoner/dist-mxnet,ZihengJiang/mxnet,jennyzhang0215/incubator-mxnet,saurabh3949/mxnet,mbaijal/incubator-mxnet,lxn2/mxnet,wangyum/mxnet,pluskid/mxnet,precedenceguo/mxnet,crazy-cat/incubator-mxnet,indhub/mxnet,mbaijal/incubator-mxnet,Guneet-Dhillon/mxnet,hotpxl/mxnet,szha/mxnet,ptrendx/mxnet,hotpxl/mxnet,larroy/mxnet,kkk669/mxnet,jamesliu/mxnet,smolix/incubator-mxnet,pluskid/mxnet,saurabh3949/mxnet,rishita/mxnet,Mega-DatA-Lab/mxnet,yajiedesign/mxnet,CodingCat/mxnet,vikingMei/mxnet,sergeykolychev/mxnet,jermainewang/mxnet,jennyzhang0215/incubator-mxnet,hesseltuinhof/mxnet,jermainewang/mxnet,Guneet-Dhillon/mxnet,yuruofeifei/mxnet,crazy-cat/incubator-mxnet,arikpoz/mxnet,szha/mxnet,sergeykolychev/mxnet,solin319/incubator-mxnet,ptrendx/mxnet,TuSimple/mxnet,pluskid/mxnet,mbaijal/incubator-mxnet,vikingMei/mxnet,reminisce/mxnet,crazy-cat/incubator-mxnet,danithaca/mxnet,TuSimple/mxnet,kkk669/mxnet,leezu/mxnet,szha/mxnet,saurabh3949/mxnet,lxn2/mxnet,CodingCat/mxnet,jiajiechen/mxnet,leezu/mxnet,EvanzzzZ/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,rahul003/mxnet,rahul003/mxnet,nicklhy/mxnet,yuruofeifei/mxnet,stefanhenneking/mxnet,ykim362/mxnet,DR08/mxnet,jennyzhang0215/incubator-mxnet,madjam/mxnet,rahul003/mxnet,hotpxl/mxnet,leezu/mxnet,yajiedesign/mxnet,danithaca/mxnet,ZihengJiang/mxnet,ForkedReposBak/mxnet,reminisce/mxnet,smolix/incubator-mxnet,dmlc/mxnet,ZihengJiang/mxnet,ptrendx/mxnet,szha/mxnet,larroy/mxnet,antoan2/incubator-mxnet,yajiedesign/mxnet,nicklhy/mxnet,antoan2/incubator-mxnet,smolix/incubator-mxnet,eric-haibin-lin/mxnet,EvanzzzZ/mxnet,arank/mxnet,TuSimple/mxnet,Guneet-Dhillon/mxnet,crazy-cat/incubator-mxnet,precedenceguo/mxnet,madjam/mxnet,ykim362/mxnet,leezu/mxnet,tornadomeet/mxnet,Northrend/mxnet,formath/mxnet,jiajiechen/mxnet,crazy-cat/incubator-mxnet,xcgoner/dist-mxnet,yuruofeifei/mxnet,hpi-xnor/BMXNet,hpi-xnor/BMXNet,arikpoz/mxnet,sxjscience/mxnet,Prasad9/incubator-mxnet,EvanzzzZ/mxnet,Ldpe2G/mxnet,Prasad9/incubator-mxnet,DR08/mxnet,dmlc/mxnet,larroy/mxnet,hpi-xnor/BMXNet,likelyzhao/mxnet,precedenceguo/mxnet,navrasio/mxnet,apaleyes/mxnet,jennyzhang0215/incubator-mxnet,navrasio/mxnet,coder-james/mxnet,kevinthesun/mxnet,eric-haibin-lin/mxnet,DickJC123/mxnet,indhub/mxnet,solin319/incubator-mxnet,fullfanta/mxnet,stefanhenneking/mxnet,sergeykolychev/mxnet,rishita/mxnet,Mega-DatA-Lab/mxnet,yuruofeifei/mxnet,tornadomeet/mxnet,precedenceguo/mxnet,coder-james/mxnet,madjam/mxnet,jamesliu/mxnet,luoyetx/mxnet,jamesliu/mxnet,danithaca/mxnet,sxjscience/mxnet,Mega-DatA-Lab/mxnet,jiajiechen/mxnet,ykim362/mxnet,likelyzhao/mxnet,jermainewang/mxnet,fullfanta/mxnet,ykim362/mxnet,hpi-xnor/BMXNet,kevinthesun/mxnet,pluskid/mxnet,pluskid/mxnet,zhreshold/mxnet,gautamkmr/incubator-mxnet,navrasio/mxnet,rahul003/mxnet,ykim362/mxnet,lxn2/mxnet,sergeykolychev/mxnet,LinkHS/incubator-mxnet,luoyetx/mxnet,eric-haibin-lin/mxnet,jermainewang/mxnet,ShownX/incubator-mxnet,crazy-cat/incubator-mxnet,tlby/mxnet,thirdwing/mxnet,wangyum/mxnet,sergeykolychev/mxnet,Mega-DatA-Lab/mxnet,indhub/mxnet,larroy/mxnet,jermainewang/mxnet,zhreshold/mxnet,gautamkmr/incubator-mxnet,indhub/mxnet,solin319/incubator-mxnet,navrasio/mxnet,kevinthesun/mxnet,rahul003/mxnet,arikpoz/mxnet,likelyzhao/mxnet,ZihengJiang/mxnet,arank/mxnet,apaleyes/mxnet,eric-haibin-lin/mxnet,ptrendx/mxnet,tlby/mxnet,lxn2/mxnet,smolix/incubator-mxnet,yajiedesign/mxnet,stefanhenneking/mxnet,CodingCat/mxnet,precedenceguo/mxnet,formath/mxnet,CodingCat/mxnet,DR08/mxnet,ptrendx/mxnet,wangyum/mxnet,reminisce/mxnet,saurabh3949/mxnet,rishita/mxnet,TuSimple/mxnet,kevinthesun/mxnet,jamesliu/mxnet,hpi-xnor/BMXNet,LinkHS/incubator-mxnet,thirdwing/mxnet,kkk669/mxnet,Ldpe2G/mxnet,ShownX/incubator-mxnet,zhreshold/mxnet,xcgoner/dist-mxnet,Guneet-Dhillon/mxnet,indhub/mxnet,xcgoner/dist-mxnet,stefanhenneking/mxnet,hpi-xnor/BMXNet,reminisce/mxnet,likelyzhao/mxnet,kkk669/mxnet,saurabh3949/mxnet,kevinthesun/mxnet,vikingMei/mxnet,precedenceguo/mxnet,tlby/mxnet,hesseltuinhof/mxnet,sxjscience/mxnet,luoyetx/mxnet,smolix/incubator-mxnet,danithaca/mxnet,smolix/incubator-mxnet,apache/incubator-mxnet,mbaijal/incubator-mxnet,jennyzhang0215/incubator-mxnet,yajiedesign/mxnet,hesseltuinhof/mxnet,tornadomeet/mxnet,weleen/mxnet,DR08/mxnet,rishita/mxnet,antoan2/incubator-mxnet,solin319/incubator-mxnet,likelyzhao/mxnet,weleen/mxnet,reminisce/mxnet,pluskid/mxnet,Mega-DatA-Lab/mxnet,jamesliu/mxnet,tlby/mxnet,Prasad9/incubator-mxnet,zhreshold/mxnet,hotpxl/mxnet,Ldpe2G/mxnet,nicklhy/mxnet,ShownX/incubator-mxnet,thirdwing/mxnet,rishita/mxnet,crazy-cat/incubator-mxnet,xcgoner/dist-mxnet,yajiedesign/mxnet,lxn2/mxnet,sergeykolychev/mxnet,wangyum/mxnet,stefanhenneking/mxnet,sxjscience/mxnet,crazy-cat/incubator-mxnet,antoan2/incubator-mxnet,coder-james/mxnet,arikpoz/mxnet,danithaca/mxnet,hotpxl/mxnet,wangyum/mxnet,rahul003/mxnet,ZihengJiang/mxnet,gautamkmr/incubator-mxnet,pluskid/mxnet,solin319/incubator-mxnet,danithaca/mxnet,jiajiechen/mxnet,ForkedReposBak/mxnet,rahul003/mxnet,ShownX/incubator-mxnet,arikpoz/mxnet,formath/mxnet,ShownX/incubator-mxnet,Prasad9/incubator-mxnet,jermainewang/mxnet,kkk669/mxnet,eric-haibin-lin/mxnet,lxn2/mxnet,kkk669/mxnet,Mega-DatA-Lab/mxnet,arank/mxnet,yuruofeifei/mxnet,jiajiechen/mxnet,larroy/mxnet,gautamkmr/incubator-mxnet,LinkHS/incubator-mxnet,hesseltuinhof/mxnet,tlby/mxnet,vikingMei/mxnet,luoyetx/mxnet,navrasio/mxnet,reminisce/mxnet,fullfanta/mxnet,ykim362/mxnet,Ldpe2G/mxnet,nicklhy/mxnet,eric-haibin-lin/mxnet,EvanzzzZ/mxnet,arank/mxnet,nicklhy/mxnet,hesseltuinhof/mxnet,Ldpe2G/mxnet,ForkedReposBak/mxnet,danithaca/mxnet,saurabh3949/mxnet,EvanzzzZ/mxnet,ForkedReposBak/mxnet,apaleyes/mxnet,hesseltuinhof/mxnet,thirdwing/mxnet,ZihengJiang/mxnet,navrasio/mxnet,arikpoz/mxnet,fullfanta/mxnet,solin319/incubator-mxnet,smolix/incubator-mxnet,arikpoz/mxnet,tlby/mxnet,arank/mxnet,dmlc/mxnet,ZihengJiang/mxnet,xcgoner/dist-mxnet,tornadomeet/mxnet,jennyzhang0215/incubator-mxnet,mbaijal/incubator-mxnet,jennyzhang0215/incubator-mxnet,DR08/mxnet,antoan2/incubator-mxnet,TuSimple/mxnet,formath/mxnet,wangyum/mxnet,arank/mxnet,ZihengJiang/mxnet,antoan2/incubator-mxnet,weleen/mxnet,Prasad9/incubator-mxnet,likelyzhao/mxnet,danithaca/mxnet,apaleyes/mxnet,likelyzhao/mxnet,indhub/mxnet,tornadomeet/mxnet,pluskid/mxnet,apaleyes/mxnet,Ldpe2G/mxnet,thirdwing/mxnet,Prasad9/incubator-mxnet,indhub/mxnet,navrasio/mxnet,precedenceguo/mxnet,indhub/mxnet,larroy/mxnet,rishita/mxnet,apache/incubator-mxnet,tornadomeet/mxnet,xcgoner/dist-mxnet,sergeykolychev/mxnet,Mega-DatA-Lab/mxnet,Mega-DatA-Lab/mxnet,fullfanta/mxnet,weleen/mxnet,weleen/mxnet,DR08/mxnet,CodingCat/mxnet,sergeykolychev/mxnet,pluskid/mxnet,szha/mxnet,zhreshold/mxnet,EvanzzzZ/mxnet,sxjscience/mxnet,yuruofeifei/mxnet,wangyum/mxnet,ZihengJiang/mxnet,lxn2/mxnet,Prasad9/incubator-mxnet,kkk669/mxnet,Northrend/mxnet,EvanzzzZ/mxnet,szha/mxnet,reminisce/mxnet,ShownX/incubator-mxnet,gautamkmr/incubator-mxnet,CodingCat/mxnet,formath/mxnet,Northrend/mxnet,Guneet-Dhillon/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,jennyzhang0215/incubator-mxnet,TuSimple/mxnet,kevinthesun/mxnet,tornadomeet/mxnet,TuSimple/mxnet,arank/mxnet,larroy/mxnet,ForkedReposBak/mxnet,hesseltuinhof/mxnet,rahul003/mxnet,wangyum/mxnet,stefanhenneking/mxnet,wangyum/mxnet,szha/mxnet,madjam/mxnet,sxjscience/mxnet,fullfanta/mxnet,solin319/incubator-mxnet,LinkHS/incubator-mxnet,luoyetx/mxnet,kkk669/mxnet,zhreshold/mxnet,madjam/mxnet,leezu/mxnet,piiswrong/mxnet,coder-james/mxnet,TuSimple/mxnet,nicklhy/mxnet,TuSimple/mxnet,formath/mxnet,apaleyes/mxnet,leezu/mxnet,ForkedReposBak/mxnet,precedenceguo/mxnet,ptrendx/mxnet,arikpoz/mxnet,luoyetx/mxnet,jamesliu/mxnet,tlby/mxnet,piiswrong/mxnet,thirdwing/mxnet,jermainewang/mxnet,ykim362/mxnet,nicklhy/mxnet,piiswrong/mxnet,likelyzhao/mxnet,saurabh3949/mxnet,tornadomeet/mxnet,jermainewang/mxnet,ptrendx/mxnet,crazy-cat/incubator-mxnet,thirdwing/mxnet,lxn2/mxnet,reminisce/mxnet,yuruofeifei/mxnet,lxn2/mxnet,fullfanta/mxnet,ykim362/mxnet,hpi-xnor/BMXNet,TuSimple/mxnet,stefanhenneking/mxnet,yuruofeifei/mxnet,tlby/mxnet,reminisce/mxnet,nicklhy/mxnet,precedenceguo/mxnet,Prasad9/incubator-mxnet,mbaijal/incubator-mxnet,likelyzhao/mxnet,madjam/mxnet,stefanhenneking/mxnet,yajiedesign/mxnet,mbaijal/incubator-mxnet,Prasad9/incubator-mxnet,Northrend/mxnet,hesseltuinhof/mxnet,arikpoz/mxnet,jamesliu/mxnet,coder-james/mxnet,dmlc/mxnet,Northrend/mxnet,rishita/mxnet,kevinthesun/mxnet,LinkHS/incubator-mxnet,gautamkmr/incubator-mxnet,weleen/mxnet,hotpxl/mxnet,jiajiechen/mxnet,gautamkmr/incubator-mxnet,thirdwing/mxnet,formath/mxnet,vikingMei/mxnet