* WIP support bn training for global_stats

(cherry picked from commit eb81a37328ea177b1d58c9eebdbb345e0fa25f0d)

* - Style fix
- Fix test case

* Addressed PR comments
- added support for bn training/inference with a same ctor
- added more verbose comments in bn header

* Fixed bn serializer and default value in bn ctor for bwd compatibility

* proposed docs change

* - Addressed PR comments
  - added support to compute bn inference/training using same mkldnn kernel with global stats

* fix unit bn relu unit test