Loss Constants: _coord, _obj and _noobj

[glenn-jocher]

The correct YOLO v3 loss constants are:

lambda_coord = 1.0
lambda_obj = 1.0
lambda_noobj = 1.0

rather than the below constants, which derive from the original yolov1.cfg file:
https://github.com/pjreddie/darknet/blob/61c9d02ec461e30d55762ec7669d6a1d3c356fb2/cfg/yolov1.cfg#L257-L260

lambda_coord = 5.0
lambda_obj = 1.0
lambda_noobj = 0.5

The latest yolov3 constants appear to be hard coded into the parser.c rather than in yolov3.cfg Credit to @ydixon who originally noticed this discrepancy in issue #12.
https://github.com/pjreddie/darknet/blob/680d3bde1924c8ee2d1c1dea54d3e56a05ca9a26/src/parser.c#L376-L381

[glenn-jocher]

Corrected in commit cf9b4cf.