updated comments for mnist tutorial (#9104)
updated comments for mnist tutorial (#9104)

* updated comments

corrected comment for cpu/gpu usage, added assertion messages

* Update mnist.md

added empty line to retrigger build.

indhub/mxnet,reminisce/mxnet,navrasio/mxnet,LinkHS/incubator-mxnet,weleen/mxnet,reminisce/mxnet,reminisce/mxnet,szha/mxnet,eric-haibin-lin/mxnet,rahul003/mxnet,reminisce/mxnet,jiajiechen/mxnet,jennyzhang0215/incubator-mxnet,sxjscience/mxnet,indhub/mxnet,luoyetx/mxnet,ForkedReposBak/mxnet,navrasio/mxnet,Northrend/mxnet,LinkHS/incubator-mxnet,larroy/mxnet,mbaijal/incubator-mxnet,apache/incubator-mxnet,larroy/mxnet,jamesliu/mxnet,zhreshold/mxnet,navrasio/mxnet,TuSimple/mxnet,precedenceguo/mxnet,tornadomeet/mxnet,tornadomeet/mxnet,tlby/mxnet,TuSimple/mxnet,navrasio/mxnet,jiajiechen/mxnet,tornadomeet/mxnet,luoyetx/mxnet,TuSimple/mxnet,sxjscience/mxnet,ptrendx/mxnet,sxjscience/mxnet,Northrend/mxnet,precedenceguo/mxnet,dmlc/mxnet,zhreshold/mxnet,jamesliu/mxnet,mbaijal/incubator-mxnet,ForkedReposBak/mxnet,tlby/mxnet,yajiedesign/mxnet,indhub/mxnet,ptrendx/mxnet,rahul003/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,mbaijal/incubator-mxnet,fullfanta/mxnet,jiajiechen/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,luoyetx/mxnet,szha/mxnet,szha/mxnet,tlby/mxnet,larroy/mxnet,precedenceguo/mxnet,mbaijal/incubator-mxnet,jiajiechen/mxnet,vikingMei/mxnet,jamesliu/mxnet,dmlc/mxnet,jamesliu/mxnet,precedenceguo/mxnet,leezu/mxnet,yajiedesign/mxnet,LinkHS/incubator-mxnet,weleen/mxnet,leezu/mxnet,sxjscience/mxnet,ptrendx/mxnet,apache/incubator-mxnet,ForkedReposBak/mxnet,szha/mxnet,vikingMei/mxnet,szha/mxnet,indhub/mxnet,indhub/mxnet,tlby/mxnet,CodingCat/mxnet,dmlc/mxnet,vikingMei/mxnet,navrasio/mxnet,TuSimple/mxnet,zhreshold/mxnet,apache/incubator-mxnet,weleen/mxnet,navrasio/mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,indhub/mxnet,CodingCat/mxnet,leezu/mxnet,leezu/mxnet,rahul003/mxnet,reminisce/mxnet,dmlc/mxnet,rahul003/mxnet,jamesliu/mxnet,sxjscience/mxnet,zhreshold/mxnet,larroy/mxnet,weleen/mxnet,rahul003/mxnet,vikingMei/mxnet,LinkHS/incubator-mxnet,reminisce/mxnet,rahul003/mxnet,TuSimple/mxnet,tornadomeet/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,jiajiechen/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,tlby/mxnet,tlby/mxnet,CodingCat/mxnet,Northrend/mxnet,indhub/mxnet,luoyetx/mxnet,navrasio/mxnet,szha/mxnet,Northrend/mxnet,apache/incubator-mxnet,szha/mxnet,precedenceguo/mxnet,weleen/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,LinkHS/incubator-mxnet,sxjscience/mxnet,weleen/mxnet,LinkHS/incubator-mxnet,ptrendx/mxnet,jennyzhang0215/incubator-mxnet,fullfanta/mxnet,tlby/mxnet,luoyetx/mxnet,ForkedReposBak/mxnet,mbaijal/incubator-mxnet,ptrendx/mxnet,jamesliu/mxnet,TuSimple/mxnet,larroy/mxnet,indhub/mxnet,dmlc/mxnet,weleen/mxnet,yajiedesign/mxnet,szha/mxnet,vikingMei/mxnet,precedenceguo/mxnet,vikingMei/mxnet,leezu/mxnet,jiajiechen/mxnet,jennyzhang0215/incubator-mxnet,LinkHS/incubator-mxnet,szha/mxnet,yajiedesign/mxnet,rahul003/mxnet,sxjscience/mxnet,dmlc/mxnet,LinkHS/incubator-mxnet,larroy/mxnet,eric-haibin-lin/mxnet,jennyzhang0215/incubator-mxnet,rahul003/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,jennyzhang0215/incubator-mxnet,DickJC123/mxnet,eric-haibin-lin/mxnet,dmlc/mxnet,LinkHS/incubator-mxnet,tornadomeet/mxnet,leezu/mxnet,rahul003/mxnet,CodingCat/mxnet,Northrend/mxnet,larroy/mxnet,reminisce/mxnet,fullfanta/mxnet,TuSimple/mxnet,CodingCat/mxnet,DickJC123/mxnet,jiajiechen/mxnet,luoyetx/mxnet,yajiedesign/mxnet,DickJC123/mxnet,zhreshold/mxnet,dmlc/mxnet,tornadomeet/mxnet,larroy/mxnet,ptrendx/mxnet,ptrendx/mxnet,jiajiechen/mxnet,ptrendx/mxnet,tornadomeet/mxnet,yajiedesign/mxnet,navrasio/mxnet,precedenceguo/mxnet,ptrendx/mxnet,Northrend/mxnet,eric-haibin-lin/mxnet,vikingMei/mxnet,tlby/mxnet,luoyetx/mxnet,larroy/mxnet,Northrend/mxnet,yajiedesign/mxnet,fullfanta/mxnet,weleen/mxnet,indhub/mxnet,Northrend/mxnet,navrasio/mxnet,CodingCat/mxnet,Northrend/mxnet,weleen/mxnet,luoyetx/mxnet,ptrendx/mxnet,fullfanta/mxnet,ForkedReposBak/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,sxjscience/mxnet,reminisce/mxnet,zhreshold/mxnet,fullfanta/mxnet,TuSimple/mxnet,jennyzhang0215/incubator-mxnet,jamesliu/mxnet,leezu/mxnet,yajiedesign/mxnet,vikingMei/mxnet,precedenceguo/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,jennyzhang0215/incubator-mxnet,DickJC123/mxnet,vikingMei/mxnet,TuSimple/mxnet,CodingCat/mxnet,mbaijal/incubator-mxnet,sxjscience/mxnet,tornadomeet/mxnet,mbaijal/incubator-mxnet,precedenceguo/mxnet,yajiedesign/mxnet,fullfanta/mxnet,fullfanta/mxnet,jamesliu/mxnet,jiajiechen/mxnet,tornadomeet/mxnet,mbaijal/incubator-mxnet,reminisce/mxnet,CodingCat/mxnet,ForkedReposBak/mxnet,jennyzhang0215/incubator-mxnet,dmlc/mxnet,fullfanta/mxnet,zhreshold/mxnet,luoyetx/mxnet,jamesliu/mxnet