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.

* Added kernel to is_empty_op

* Added python API

* Updated code as required

* Updated unittests

* listen_and_serv use localscope

* fix ut

- Finished draft of pooling reusing of operators

- Using gethash in PoolGrad added

- Removed diagnostic

- Added pool mkldnn grad reusing of primitives

- Added diagnostic

- Removed diagnostic

- added dependency to mkldnn data type for pooling mkldnn

- Added mkldnn memory data type determining based on template type of op

- Compilation warning fix

- codying style fixes

* fix cmake error

* update fix

* Fix a bug related to dispensable inputs and refine the inference unittest.

* Fix the use of dispensable inputs in reshape_op.

* Polish the enforce statements.

* Fix an English writing typo.

- Added hash function inside of MKLDNN softmax op to be used as handle for primitives stroing in a
context

- Style fixes to softmax mkldnn op

- Fixes after review

- Coding style

- Fix to style

- style fixes

- style fix

- style fixes

- Fix to cody style check

- Rephrasing a comment

* Add FP16 option in save_combine_op

* Fix casting issue