use mx.context.num_gpus instead of mx.test_utils.list_gpus in MF recommender example (#14926)
use mx.context.num_gpus instead of mx.test_utils.list_gpus in MF recommender example (#14926)


szha/mxnet,leezu/mxnet,zhreshold/mxnet,zhreshold/mxnet,larroy/mxnet,szha/mxnet,zhreshold/mxnet,sxjscience/mxnet,ForkedReposBak/mxnet,ForkedReposBak/mxnet,eric-haibin-lin/mxnet,reminisce/mxnet,leezu/mxnet,szha/mxnet,leezu/mxnet,zhreshold/mxnet,yajiedesign/mxnet,szha/mxnet,tlby/mxnet,zhreshold/mxnet,tlby/mxnet,larroy/mxnet,reminisce/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,apache/incubator-mxnet,leezu/mxnet,yajiedesign/mxnet,szha/mxnet,reminisce/mxnet,reminisce/mxnet,ForkedReposBak/mxnet,ForkedReposBak/mxnet,DickJC123/mxnet,tlby/mxnet,leezu/mxnet,larroy/mxnet,sxjscience/mxnet,apache/incubator-mxnet,leezu/mxnet,ForkedReposBak/mxnet,leezu/mxnet,eric-haibin-lin/mxnet,tlby/mxnet,DickJC123/mxnet,szha/mxnet,eric-haibin-lin/mxnet,szha/mxnet,szha/mxnet,yajiedesign/mxnet,zhreshold/mxnet,ForkedReposBak/mxnet,larroy/mxnet,tlby/mxnet,larroy/mxnet,ForkedReposBak/mxnet,larroy/mxnet,tlby/mxnet,yajiedesign/mxnet,ForkedReposBak/mxnet,reminisce/mxnet,szha/mxnet,apache/incubator-mxnet,reminisce/mxnet,apache/incubator-mxnet,zhreshold/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,sxjscience/mxnet,DickJC123/mxnet,leezu/mxnet,reminisce/mxnet,sxjscience/mxnet,reminisce/mxnet,DickJC123/mxnet,reminisce/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,eric-haibin-lin/mxnet,reminisce/mxnet,yajiedesign/mxnet,leezu/mxnet,sxjscience/mxnet,eric-haibin-lin/mxnet,sxjscience/mxnet,zhreshold/mxnet,sxjscience/mxnet,eric-haibin-lin/mxnet,ForkedReposBak/mxnet,sxjscience/mxnet,zhreshold/mxnet,larroy/mxnet,yajiedesign/mxnet,tlby/mxnet,larroy/mxnet,larroy/mxnet,yajiedesign/mxnet,sxjscience/mxnet,yajiedesign/mxnet,yajiedesign/mxnet