It can be run both CPU/GPU. configure attributes are:

* min: the min value of uniform random
* max: the max value of uniform random
* dims: the dimension of output tensor
* seed: the random seed of uniform random. 0 means generate a seed each
        time.

* init cross entropy graident

* add crossentropy grad op

* remove details

* fix static compile

* move namespace rnn to a directory

* User must install Paddle python package before unittest.
* Or use docker to build Paddle

* because it uses `numpy.flip` in `test_image.py`