fix bug of not using get_score_cls_index in BoxWithNMSLimitOp (#20868)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20868
When `input_boxes_include_bg_cls` is false (which means `input_scores_fg_cls_starting_id` is 0), It doesn't map the class index of score currectly when sorting and limiting the detections over all classes after nms.
Reviewed By: newstzpz
Differential Revision: D15472706
fbshipit-source-id: dc1e808b63ad09fb4bd95acf866771bb3fa92d69
