Allow for images to contain zero true detections (#1531)

* Allow for images to contain zero true detections

* Allow for empty assignment in PointAssigner

* Allow ApproxMaxIouAssigner to return an empty result

* Fix CascadeRNN forward when entire batch has no truth

* Correctly assign boxes to background when there is no truth

* Fix assignment tests

* Make flatten robust

* Fix bbox loss with empty pred/truth

* Fix logic error in BBoxHead.loss

* Add tests for empty truth cases

* tests faster rcnn empty forward

* Skip roipool forward tests if torchvision is not installed

* Add tests for bbox/anchor heads

* Consolidate test_forward and test_forward2

* Fix assign_results.labels = None when gt_labels is given; Add test for this case

* Fix OHEM Sampler with zero truth

* remove xdev

* resolve 3 reviews

* Fix flake8

* refactoring

* fix yaml format

* add filter flag

* minor fix

* delete redundant code in load anno

* fix flake8 errors

* quick fix for empty truth with masks

* fix yapf error

* fix mask padding for empty masks

Co-authored-by: Cao Yuhang <yhcao6@gmail.com>
Co-authored-by: Kai Chen <chenkaidev@gmail.com>