* fix acc and iou compute nan problem

* fix acc and iou compute nan problem

* add nan_to_num args for mean_iou

* add nan_to_num args for mean_iou

* add nan_to_num args for mean_iou

* add nan_to_num args for mean_iou

* add nan_to_num args for mean_iou

* Update mmseg/core/evaluation/mean_iou.py

* Update mean_iou.py

* Update mean_iou.py

Co-authored-by: Jerry Jiarui XU <xvjiarui0826@gmail.com>