fix PrefetchingIter for provide_data/label (#3393)
fix PrefetchingIter for provide_data/label (#3393)

* fix PrefetchingIter for provide_data/label

* fix lint

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