Fix block dropout on GPU
Fix block dropout on GPU

danielhers/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,clab/dynet,clab/dynet,shuheik/dynet,shuheik/dynet,clab/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,danielhers/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,shuheik/dynet,clab/cnn,clab/dynet,clab/dynet,shuheik/dynet,clab/dynet,danielhers/dynet,clab/dynet