Made CUDA choose device with most free memory by default
Made CUDA choose device with most free memory by default


Former-commit-id: 2211f1f2028eb3f7d739f3673135b685f132d903
clab/dynet,clab/dynet,shuheik/dynet,xunzhang/dynet,clab/cnn,clab/cnn,shuheik/dynet,xunzhang/dynet,shuheik/dynet,shuheik/dynet,danielhers/dynet,danielhers/dynet,danielhers/dynet,xunzhang/dynet,chunyang-wen/dynet,clab/dynet,chunyang-wen/dynet,danielhers/dynet,clab/dynet,shuheik/dynet,chunyang-wen/dynet,shuheik/dynet,clab/cnn,clab/dynet,danielhers/dynet,chunyang-wen/dynet,danielhers/dynet,shuheik/dynet,clab/dynet,clab/dynet,xunzhang/dynet,danielhers/dynet