[BUG] `current_segment_id` should increment from largest segment id.
Code below increment segment id from last iteration segment id, which is wrong. Correct segment id should increment from the latest(which is the largest so far) segment id.
https://github.com/huggingface/transformers/blob/a7920065f2cfd2549b838f9a30afd7c265fcdd88/src/transformers/models/mask2former/image_processing_mask2former.py#L234-L237
Hi @jimmysue, thanks for raising this issue! 

For the segment ids, they increment from 0 -> total number of segments for each image. This is expected for segmentation like panoptic, as we want each individual instance to have its own segment ID assigned e.g. segment 0 -> car 0, segment 1 -> car 1, segment 3 -> sky etc. 
