SINGA-387 Modified the design of autograd backward engine and correct some mistakes in it
SINGA-387 Modified the design of autograd backward engine and correct some mistakes in it

another solution:
- let Dummy involved in dependency counting system
- modified former backward function

nudles/incubator-singa,nusdbsystem/incubator-singa,apache/incubator-singa,ijingo/incubator-singa,apache/incubator-singa,kaiping/incubator-singa,nudles/incubator-singa,kaiping/incubator-singa,ijingo/incubator-singa,nusdbsystem/incubator-singa,kaiping/incubator-singa,apache/incubator-singa,nudles/incubator-singa,kaiping/incubator-singa,apache/incubator-singa,ijingo/incubator-singa,nusdbsystem/incubator-singa,nudles/incubator-singa,ijingo/incubator-singa,nusdbsystem/incubator-singa,apache/incubator-singa,nudles/incubator-singa,kaiping/incubator-singa,ijingo/incubator-singa,nusdbsystem/incubator-singa