[dynet-1018]. Avoid allocate CPU memory under GPU mode. (#1024)
[dynet-1018]. Avoid allocate CPU memory under GPU mode. (#1024)

* Avoid allocate CPU memory under GPU mode

* fix dismatch type

* modify default mem size


Former-commit-id: c6420c2adc88b94180ae12028cc89cd9a845e943
clab/cnn,xunzhang/dynet,clab/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,clab/cnn,danielhers/dynet,danielhers/dynet,xunzhang/dynet,shuheik/dynet,clab/dynet,clab/dynet,danielhers/dynet,danielhers/dynet,clab/cnn,xunzhang/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,shuheik/dynet,danielhers/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,chunyang-wen/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet