externalize w1dt computation and cache it
externalize w1dt computation and cache it


Former-commit-id: 991aa50757b094714c776cd8ee40645e957b7ffd
clab/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,clab/dynet,shuheik/dynet,clab/dynet,clab/cnn,shuheik/dynet,chunyang-wen/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,clab/dynet,clab/dynet,shuheik/dynet,danielhers/dynet,shuheik/dynet,chunyang-wen/dynet,shuheik/dynet,chunyang-wen/dynet,shuheik/dynet,shuheik/dynet,xunzhang/dynet,clab/dynet,clab/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/cnn,xunzhang/dynet,danielhers/dynet,danielhers/dynet