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

* fix wheel package name

* delete cuda7.5 cudnn5

* Added kernel to is_empty_op

* Added python API

* Updated code as required

* Updated unittests

* "a piece of job."

* "fix typeo"

* "fix ci"

add title for cpu_profiling.md

Reuse of pooling mkldnn primitives

Fix a compile error

* package prior_box op

* add doc

* add unittest

* add unittest

* fix CI fails

* Dockerfile: unify dev and product docker environment.

* Fix check style error.

* Use envrioment valuable to get version info.

Fix a profiler race condition

* use latest pip version

* fix ci

* update by comment

allow inference test to generate timeline

Inferencer support parallel_executor