fixes workspace being bigger than anticipated
fixes workspace being bigger than anticipated

The workspace required is calculated in the end as
`shape_colunit_.Size() * nstep_ + shape_dstunit_.Size() * nstep_` in
order to stay within the memory requirements given by workspace `nstep` must be
also calculated taking into account the memory requirement for `dstunit`.
Otherwise for large `shape_dstunit_.Size()`, the code would
underestimate the memory requirements for `dstunit`.

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