* Add RetinetNet parameters in cfg.

* hot fix.

* Add the retinanet head module now.

* Add the function to generate the anchors for RetinaNet.

* Add the SigmoidFocalLoss cuda operator.

* Fix the bug in the extra layers.

* Change the normalizer for SigmoidFocalLoss

* Support multiscale in training.

* Add retinannet  training script.

* Add the inference part of RetinaNet.

* Fix the bug when building the extra layers in retinanet.
Update the matching part in retinanet_loss.

* Add the first version of the inference of RetinaNet.
Need to check it again to see if is there any room for speed
improvement.

* Remove the  retinanet_R-50-FPN_2x.yaml first.

* Optimize the retinanet postprocessing.

* quick fix.

* Add script for training RetinaNet with ResNet101 backbone.

* Move cfg.RETINANET to cfg.MODEL.RETINANET

* Remove the variables which are not used.

* revert boxlist_ops.
Generate Empty BoxLists instead of [] in retinanet_infer

* Remove the not used commented lines.
Add NUM_DETECTIONS_PER_IMAGE

* remove the not used codes.

* Move retinanet related files under Modeling/rpn/retinanet

* Add retinanet_X_101_32x8d_FPN_1x.yaml script.
This model is not fully validated. I only trained it around 5000
iterations and everything is fine.

* set RETINANET.PRE_NMS_TOP_N as 0 in level5 (p7), because previous setting may generate zero detections and could cause
the program break.
This part is used in original Detectron setting.

* Fix the rpn only bug when the training ends.

* Minor improvements

* Comments and add Python-only implementation

* Bugfix and remove commented code

* keep the generalized_rcnn same.
Move the build_retinanet inside build_rpn.

* Add USE_C5 in the MODEL.RETINANET

* Add two configs using P5 to generate P6.

* fix the bug when loading the Caffe2 ImageNet pretrained model.

* Reduce the code depulication of RPN loss and RetinaNet loss.

* Remove the comment which is not used.

* Remove the hard coded number of classes.

* share the foward part of rpn inference.

* fix the bug in rpn inference.

* Remove the conditional part in the inference.

* Bug fix: add the utils file for permute and flatten of the box
prediction layers.

* Update the comment.

* quick fix. Adding import cat.

* quick fix: forget including import.

* Adjust the normalization part according to Detectron's setting.

* Use the bbox reg normalization term.

* Clean the code according to recent review.

* Using CUDA version for training now. And the python version for training
on cpu.

* rename the directory to retinanet.

* Make the train and val datasets are consistent with mask r-cnn setting.

* add comment.