updated backprop


Former-commit-id: e3822604a4ba79411bc975ce93dd4062d82fc830
working backprop; noticable difference in training error now


Former-commit-id: 5b5bca57a746d6e7a65a1d43dadbf6a229ca13aa [formerly 8fb7e972c7b1da75744b4ecc940635154a5106f9]
Former-commit-id: 612740f1edecdc1a149f90003f3fbbd837b380b5
guard against numerical underflow; also a few fixes in sda


Former-commit-id: 6a130830b638b016a0d1fe4f5555b8e97b7cd8a1 [formerly 215b9152cb947e04a73f3f372489b8727c603f1a]
Former-commit-id: 31bf10eb5ae6760ba4fc3135d6c759976c79a335
updated


Former-commit-id: dc6f900bfe5e7aa31589d5dd87a4f6a4e69fc8ba [formerly a8f0f75f99f826fd3766f6dc8f22634236636f4c]
Former-commit-id: 4a0381745d32f38ac3b1eea9b6925ebf76ce9bb1
update


Former-commit-id: 5ec4529a363d090d28d9f66b0e19c1ba47916887