Do not use ctor
* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.
* remove net op and cond_op
* fix cpplint
* fix dependency
* delete backward_test; fix compile
* disable batch_norm backward
* rm test_net.py
* make batchnorm test independent of backward.cc
* make test_layer_norm_op independent of backward.cc
* make test_layer_norm_op independent of backward.cc
* delete unused code
* clean up