[WIP] Add SHIFT reference evaluators

[suniique]

Task	Metrics	Status
Semantic segmentation	mIoU, mAcc, fwIoU, fwAcc	[x]
Object detection 2D	COCO (mAP, AP50 AP75)	[x]
Instance segmentation	COCO (mAP, AP50 AP75)	[x]
Depth estimation	MAE, Delta_{1,2,5}, SILog	[x]
Optical flow	EPE	[x]
MOT	MOTA, HOTA, IDSw	[ ]
Object detection 3D	nuScene (mAP, AP50 AP75)	[ ]
MOT 3D	nuScene (MOTA, HOTA, IDSw)	[ ]
Trajectory forecasting	EPE, APE	[ ]
Pose estimation (human)	PCK	[ ]
Dump to eval.ai format		[x]