* Fix mAP evaluator bug.

* Fix bug in detection mAP evaluator.

* Fix unit testing.

* Support to set background label index in detection mAP op.

Implement detection mAP evaluator wrapper and unify label format between SSD loss and mAP evaluator  (#8736)

* Implement mAP evalutor Python interface.

* Fix unit testing and uniy label format between SSD loss and mAP evalutor.

* Update doc.

* fix ssd problems

* fix box decoder op

* fix dimension problem in detection tests

* update detection doc

* Update detection doc

* Update detection doc

* update detection doc

* update detection doc

* Implement multibox loss wrapper in Python API.

* Add some wrappers for SSD detection.

* Fix conflicts.

* Add unit testing for SSD loss wrapper.

* Update doc in Python API.

* Refine unit testing.

* Add more unit testing and update some interface arguments.