perform training on negative samples (images with no annotation)

[gcheron]

@ir413 , @JiYuanFeng , this allows the use of negative samples in training when setting TRAIN.INCLUDE_BKG_IMAGES = True (related to #697 )

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[pkhigh]

@gcheron I have tried to train a RETINANET model with your pr_negative_samples branch.
But I get the following error while loading the data :

[I net_dag_utils.cc:102] Operator graph pruning prior to chain compute took: 0.000376701 secs
INFO train.py: 180: Outputs saved to: /home/roadzen/ParasWork/MobileData/v4/RETINA_MODELS/train/coco_2014_train/retinanet
INFO loader.py: 230: Pre-filling mini-batch queue...
INFO loader.py: 235: [0/64]
Traceback (most recent call last):
File "/home/roadzen/ParasWork/Detectron/detectron/utils/coordinator.py", line 50, in stop_on_exception
yield
File "/home/roadzen/ParasWork/Detectron/detectron/roi_data/loader.py", line 101, in minibatch_loader_thread
blobs = self.get_next_minibatch()
File "/home/roadzen/ParasWork/Detectron/detectron/roi_data/loader.py", line 134, in get_next_minibatch
blobs, valid = get_minibatch(minibatch_db)
File "/home/roadzen/ParasWork/Detectron/detectron/roi_data/minibatch.py", line 81, in get_minibatch
blobs, im_scales, roidb, im_width, im_height
File "/home/roadzen/ParasWork/Detectron/detectron/roi_data/retinanet.py", line 109, in add_retinanet_blobs
'Empty ground truth empty for image is not allowed. Please check.'
AssertionError: Empty ground truth empty for image is not allowed. Please check.
INFO loader.py: 113: Stopping mini-batch loading thread
INFO loader.py: 113: Stopping mini-batch loading thread
INFO loader.py: 113: Stopping mini-batch loading thread
INFO loader.py: 113: Stopping mini-batch loading thread
CRITICAL train.py: 98: roi_data_loader failed
Traceback (most recent call last):
File "tools/train_net.py", line 132, in
main()
File "tools/train_net.py", line 114, in main
checkpoints = detectron.utils.train.train_model()
File "/home/roadzen/ParasWork/Detectron/detectron/utils/train.py", line 64, in train_model
handle_critical_error(model, 'roi_data_loader failed')
File "/home/roadzen/ParasWork/Detectron/detectron/utils/train.py", line 100, in handle_critical_error
raise Exception(msg)
Exception: roi_data_loader failed

[pkhigh]

It looks like you have done changes for RPN.py i.e. models which use RPNs will only be able to use this feature. There are on changes in "roi_data/retinanet.py" therefore this error occurs.
Can you suggest the changes to be made in roi_data/retinanet.py so that I can train it with background images?