Convert Detection Layer from Python to TensorFlow ops.

shapes more-or-less match now. weights load. current error=InvalidArgumentError (see above for traceback): WhereOp: Unhandled input dimensions: 0

Remove py_func for cross-environment serializability. Convert DetectionLayer/refine_detections to tf

this didnt happen for me with the coco dataset itself because there were always class ids, but for another dataset it blew up.

* Remove dropout layer. Unnecessary.
* Reduce anchor stride from 2 to 1
* Increase training proposals to 200/image
* Reduce pre-NMS limit to 6000
* Improve computing proposal post:neg ratio
* Updated COCO training schedule

Reduces odds of weight explosion.