add abs support double grad for the api 2.0

* set esp as 1e-6 to solve elu unitest fail,test=develop

* support elu activation double grad,test=develop

* delete the code commit in .cc,test=develop

* fix relu test unpass, test=develop

* add elu double grad kernel and unit test

* add caculate dX in elu double grad functor, test=develop

* update the commit code,test=develop