* Add dice loss.

* Fix comments.

* Remove unused code.

Force some unittests serial

update bcebos url so that it can also be downloaded in inner network

change CMAKE_INSTALL_PREFIX in inference_lib_dist to FLUID_INSTALL_DIR

fix warpctc

enable MKLDNN inference test

Update activations for MKL-DNN

1. If all bboxes are not difficult ground truth, the users can not define the data layer for this flag and not the input can be None for detection_map API.
2. Set default value for aspect_ratios in prior_box API.
3. Add more check in prior_box API.

add cpp trainer lib and demo

Add pip install description

add version and cmakecache in inference_lib