fix misleading test_conv.py in how_to (#7208)
fix misleading test_conv.py in how_to (#7208)

tests/python/gpu/test_conv.py is not available anymore and is refeering to the mnist example mxnet/example/image-classification
see https://github.com/dmlc/mxnet/issues/4806
and 
https://github.com/dmlc/mxnet/issues/7204
kevinthesun/mxnet,ZihengJiang/mxnet,DR08/mxnet,Mega-DatA-Lab/mxnet,kkk669/mxnet,hotpxl/mxnet,zhreshold/mxnet,jermainewang/mxnet,stefanhenneking/mxnet,saurabh3949/mxnet,rahul003/mxnet,zhreshold/mxnet,jermainewang/mxnet,wangyum/mxnet,TuSimple/mxnet,DickJC123/mxnet,ForkedReposBak/mxnet,formath/mxnet,navrasio/mxnet,Northrend/mxnet,nicklhy/mxnet,gautamkmr/incubator-mxnet,szha/mxnet,szha/mxnet,ykim362/mxnet,kkk669/mxnet,kevinthesun/mxnet,tlby/mxnet,hotpxl/mxnet,szha/mxnet,xcgoner/dist-mxnet,ZihengJiang/mxnet,stefanhenneking/mxnet,szha/mxnet,jiajiechen/mxnet,jermainewang/mxnet,nicklhy/mxnet,hesseltuinhof/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,jiajiechen/mxnet,nicklhy/mxnet,LinkHS/incubator-mxnet,ShownX/incubator-mxnet,leezu/mxnet,CodingCat/mxnet,sxjscience/mxnet,sxjscience/mxnet,yajiedesign/mxnet,jamesliu/mxnet,stefanhenneking/mxnet,ShownX/incubator-mxnet,kkk669/mxnet,thirdwing/mxnet,antoan2/incubator-mxnet,rahul003/mxnet,reminisce/mxnet,jermainewang/mxnet,tornadomeet/mxnet,piiswrong/mxnet,Northrend/mxnet,thirdwing/mxnet,jennyzhang0215/incubator-mxnet,solin319/incubator-mxnet,luoyetx/mxnet,hotpxl/mxnet,ptrendx/mxnet,weleen/mxnet,jiajiechen/mxnet,szha/mxnet,ForkedReposBak/mxnet,solin319/incubator-mxnet,weleen/mxnet,thirdwing/mxnet,antoan2/incubator-mxnet,formath/mxnet,ShownX/incubator-mxnet,ykim362/mxnet,ptrendx/mxnet,mbaijal/incubator-mxnet,tlby/mxnet,navrasio/mxnet,ShownX/incubator-mxnet,ShownX/incubator-mxnet,yajiedesign/mxnet,nicklhy/mxnet,yuruofeifei/mxnet,ShownX/incubator-mxnet,eric-haibin-lin/mxnet,hotpxl/mxnet,mbaijal/incubator-mxnet,ptrendx/mxnet,larroy/mxnet,LinkHS/incubator-mxnet,jamesliu/mxnet,vikingMei/mxnet,Northrend/mxnet,Mega-DatA-Lab/mxnet,thirdwing/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,yajiedesign/mxnet,luoyetx/mxnet,xcgoner/dist-mxnet,crazy-cat/incubator-mxnet,ykim362/mxnet,Mega-DatA-Lab/mxnet,precedenceguo/mxnet,hesseltuinhof/mxnet,dmlc/mxnet,precedenceguo/mxnet,dmlc/mxnet,Prasad9/incubator-mxnet,hotpxl/mxnet,thirdwing/mxnet,crazy-cat/incubator-mxnet,CodingCat/mxnet,jiajiechen/mxnet,TuSimple/mxnet,piiswrong/mxnet,LinkHS/incubator-mxnet,kkk669/mxnet,CodingCat/mxnet,jennyzhang0215/incubator-mxnet,dmlc/mxnet,sxjscience/mxnet,smolix/incubator-mxnet,kevinthesun/mxnet,hesseltuinhof/mxnet,madjam/mxnet,Mega-DatA-Lab/mxnet,hpi-xnor/BMXNet,hesseltuinhof/mxnet,mbaijal/incubator-mxnet,hotpxl/mxnet,szha/mxnet,tornadomeet/mxnet,jamesliu/mxnet,stefanhenneking/mxnet,DR08/mxnet,indhub/mxnet,Mega-DatA-Lab/mxnet,ForkedReposBak/mxnet,smolix/incubator-mxnet,Northrend/mxnet,gautamkmr/incubator-mxnet,kevinthesun/mxnet,sxjscience/mxnet,sxjscience/mxnet,antoan2/incubator-mxnet,fullfanta/mxnet,yuruofeifei/mxnet,jamesliu/mxnet,jiajiechen/mxnet,hpi-xnor/BMXNet,tornadomeet/mxnet,mbaijal/incubator-mxnet,jermainewang/mxnet,wangyum/mxnet,reminisce/mxnet,tornadomeet/mxnet,navrasio/mxnet,antoan2/incubator-mxnet,fullfanta/mxnet,DickJC123/mxnet,CodingCat/mxnet,hotpxl/mxnet,kevinthesun/mxnet,precedenceguo/mxnet,jiajiechen/mxnet,ShownX/incubator-mxnet,hotpxl/mxnet,tlby/mxnet,nicklhy/mxnet,sergeykolychev/mxnet,hesseltuinhof/mxnet,wangyum/mxnet,LinkHS/incubator-mxnet,weleen/mxnet,Northrend/mxnet,hpi-xnor/BMXNet,xcgoner/dist-mxnet,larroy/mxnet,gautamkmr/incubator-mxnet,Prasad9/incubator-mxnet,weleen/mxnet,ykim362/mxnet,formath/mxnet,kkk669/mxnet,jermainewang/mxnet,wangyum/mxnet,precedenceguo/mxnet,DR08/mxnet,ptrendx/mxnet,jennyzhang0215/incubator-mxnet,gautamkmr/incubator-mxnet,rahul003/mxnet,formath/mxnet,fullfanta/mxnet,Prasad9/incubator-mxnet,rahul003/mxnet,sxjscience/mxnet,formath/mxnet,LinkHS/incubator-mxnet,crazy-cat/incubator-mxnet,formath/mxnet,ZihengJiang/mxnet,zhreshold/mxnet,sergeykolychev/mxnet,larroy/mxnet,weleen/mxnet,jermainewang/mxnet,wangyum/mxnet,yuruofeifei/mxnet,madjam/mxnet,LinkHS/incubator-mxnet,xcgoner/dist-mxnet,reminisce/mxnet,vikingMei/mxnet,xcgoner/dist-mxnet,ZihengJiang/mxnet,jiajiechen/mxnet,stefanhenneking/mxnet,yajiedesign/mxnet,rahul003/mxnet,fullfanta/mxnet,CodingCat/mxnet,antoan2/incubator-mxnet,vikingMei/mxnet,gautamkmr/incubator-mxnet,rahul003/mxnet,xcgoner/dist-mxnet,wangyum/mxnet,navrasio/mxnet,ykim362/mxnet,indhub/mxnet,formath/mxnet,crazy-cat/incubator-mxnet,kkk669/mxnet,crazy-cat/incubator-mxnet,TuSimple/mxnet,yajiedesign/mxnet,tornadomeet/mxnet,gautamkmr/incubator-mxnet,larroy/mxnet,precedenceguo/mxnet,rahul003/mxnet,hesseltuinhof/mxnet,apache/incubator-mxnet,TuSimple/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,hesseltuinhof/mxnet,DR08/mxnet,crazy-cat/incubator-mxnet,TuSimple/mxnet,tlby/mxnet,ForkedReposBak/mxnet,luoyetx/mxnet,zhreshold/mxnet,saurabh3949/mxnet,ptrendx/mxnet,madjam/mxnet,kevinthesun/mxnet,tlby/mxnet,sergeykolychev/mxnet,sergeykolychev/mxnet,jennyzhang0215/incubator-mxnet,stefanhenneking/mxnet,dmlc/mxnet,yuruofeifei/mxnet,smolix/incubator-mxnet,piiswrong/mxnet,jermainewang/mxnet,kkk669/mxnet,rahul003/mxnet,leezu/mxnet,sergeykolychev/mxnet,saurabh3949/mxnet,zhreshold/mxnet,ptrendx/mxnet,ptrendx/mxnet,szha/mxnet,kevinthesun/mxnet,madjam/mxnet,hpi-xnor/BMXNet,tlby/mxnet,DR08/mxnet,TuSimple/mxnet,eric-haibin-lin/mxnet,Prasad9/incubator-mxnet,stefanhenneking/mxnet,nicklhy/mxnet,Mega-DatA-Lab/mxnet,ShownX/incubator-mxnet,LinkHS/incubator-mxnet,precedenceguo/mxnet,fullfanta/mxnet,larroy/mxnet,mbaijal/incubator-mxnet,saurabh3949/mxnet,kevinthesun/mxnet,ForkedReposBak/mxnet,jamesliu/mxnet,LinkHS/incubator-mxnet,TuSimple/mxnet,ForkedReposBak/mxnet,ptrendx/mxnet,indhub/mxnet,reminisce/mxnet,saurabh3949/mxnet,indhub/mxnet,solin319/incubator-mxnet,sxjscience/mxnet,crazy-cat/incubator-mxnet,jennyzhang0215/incubator-mxnet,larroy/mxnet,solin319/incubator-mxnet,ptrendx/mxnet,wangyum/mxnet,thirdwing/mxnet,ptrendx/mxnet,CodingCat/mxnet,larroy/mxnet,gautamkmr/incubator-mxnet,reminisce/mxnet,luoyetx/mxnet,jennyzhang0215/incubator-mxnet,kkk669/mxnet,formath/mxnet,yuruofeifei/mxnet,DR08/mxnet,piiswrong/mxnet,eric-haibin-lin/mxnet,luoyetx/mxnet,solin319/incubator-mxnet,reminisce/mxnet,xcgoner/dist-mxnet,szha/mxnet,madjam/mxnet,indhub/mxnet,vikingMei/mxnet,zhreshold/mxnet,gautamkmr/incubator-mxnet,weleen/mxnet,antoan2/incubator-mxnet,jamesliu/mxnet,vikingMei/mxnet,piiswrong/mxnet,sergeykolychev/mxnet,Prasad9/incubator-mxnet,madjam/mxnet,reminisce/mxnet,navrasio/mxnet,mbaijal/incubator-mxnet,hesseltuinhof/mxnet,jiajiechen/mxnet,DR08/mxnet,thirdwing/mxnet,smolix/incubator-mxnet,piiswrong/mxnet,yajiedesign/mxnet,yuruofeifei/mxnet,solin319/incubator-mxnet,ZihengJiang/mxnet,jennyzhang0215/incubator-mxnet,solin319/incubator-mxnet,CodingCat/mxnet,navrasio/mxnet,jamesliu/mxnet,Northrend/mxnet,indhub/mxnet,eric-haibin-lin/mxnet,dmlc/mxnet,fullfanta/mxnet,jamesliu/mxnet,yajiedesign/mxnet,vikingMei/mxnet,mbaijal/incubator-mxnet,mbaijal/incubator-mxnet,navrasio/mxnet,formath/mxnet,hpi-xnor/BMXNet,dmlc/mxnet,saurabh3949/mxnet,crazy-cat/incubator-mxnet,szha/mxnet,madjam/mxnet,tornadomeet/mxnet,ZihengJiang/mxnet,jamesliu/mxnet,TuSimple/mxnet,ykim362/mxnet,indhub/mxnet,weleen/mxnet,sergeykolychev/mxnet,smolix/incubator-mxnet,ZihengJiang/mxnet,Mega-DatA-Lab/mxnet,apache/incubator-mxnet,eric-haibin-lin/mxnet,tlby/mxnet,reminisce/mxnet,luoyetx/mxnet,antoan2/incubator-mxnet,navrasio/mxnet,ForkedReposBak/mxnet,fullfanta/mxnet,larroy/mxnet,ykim362/mxnet,luoyetx/mxnet,Prasad9/incubator-mxnet,LinkHS/incubator-mxnet,madjam/mxnet,smolix/incubator-mxnet,larroy/mxnet,thirdwing/mxnet,leezu/mxnet,hesseltuinhof/mxnet,precedenceguo/mxnet,yajiedesign/mxnet,dmlc/mxnet,DickJC123/mxnet,piiswrong/mxnet,Mega-DatA-Lab/mxnet,precedenceguo/mxnet,reminisce/mxnet,hpi-xnor/BMXNet,wangyum/mxnet,nicklhy/mxnet,zhreshold/mxnet,hpi-xnor/BMXNet,indhub/mxnet,xcgoner/dist-mxnet,DR08/mxnet,smolix/incubator-mxnet,fullfanta/mxnet,saurabh3949/mxnet,dmlc/mxnet,nicklhy/mxnet,tornadomeet/mxnet,hpi-xnor/BMXNet,antoan2/incubator-mxnet,Mega-DatA-Lab/mxnet,reminisce/mxnet,smolix/incubator-mxnet,madjam/mxnet,luoyetx/mxnet,sxjscience/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,ykim362/mxnet,ShownX/incubator-mxnet,saurabh3949/mxnet,vikingMei/mxnet,dmlc/mxnet,leezu/mxnet,ZihengJiang/mxnet,kevinthesun/mxnet,zhreshold/mxnet,tlby/mxnet,kkk669/mxnet,crazy-cat/incubator-mxnet,sergeykolychev/mxnet,eric-haibin-lin/mxnet,hpi-xnor/BMXNet,Prasad9/incubator-mxnet,zhreshold/mxnet,piiswrong/mxnet,yuruofeifei/mxnet,ykim362/mxnet,leezu/mxnet,DickJC123/mxnet,Prasad9/incubator-mxnet,precedenceguo/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,smolix/incubator-mxnet,stefanhenneking/mxnet,yajiedesign/mxnet,fullfanta/mxnet,hotpxl/mxnet,xcgoner/dist-mxnet,tornadomeet/mxnet,solin319/incubator-mxnet,antoan2/incubator-mxnet,sxjscience/mxnet,Northrend/mxnet,leezu/mxnet,rahul003/mxnet,stefanhenneking/mxnet,apache/incubator-mxnet,eric-haibin-lin/mxnet,saurabh3949/mxnet,weleen/mxnet,DR08/mxnet,luoyetx/mxnet,Prasad9/incubator-mxnet,tornadomeet/mxnet,navrasio/mxnet,jiajiechen/mxnet,piiswrong/mxnet,vikingMei/mxnet,nicklhy/mxnet,yuruofeifei/mxnet,solin319/incubator-mxnet,TuSimple/mxnet,CodingCat/mxnet,Northrend/mxnet,tlby/mxnet,jennyzhang0215/incubator-mxnet,thirdwing/mxnet,leezu/mxnet,ZihengJiang/mxnet,jermainewang/mxnet,sergeykolychev/mxnet,vikingMei/mxnet,wangyum/mxnet,apache/incubator-mxnet,gautamkmr/incubator-mxnet,yuruofeifei/mxnet,Northrend/mxnet,indhub/mxnet,weleen/mxnet,leezu/mxnet