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.

* Temporarily fix bug for backward tanspiler when using parallel_do operator.

* Fix bug for backward tanspiler when using parallel_do operator

* Update tensor_util.h

* Update with moved TensorDesc

* Fix tensur_utils.cu

* Update

* Update

* Update

* Update

* Make tensor_util.cu a symbolic link

* Implement multibox loss wrapper in Python API.

* Add some wrappers for SSD detection.

* Fix conflicts.

* Add unit testing for SSD loss wrapper.

* Update doc in Python API.

* Refine unit testing.

* Add more unit testing and update some interface arguments.