Add example dataset for MMDet3D

[suniique]

Add an example dataset API for mmdet3d (v1.0+). It supports monocular tasks, including 2D/3D detection, tracking, 2D instance segmentation, and depth estimation.

Usage:

dict(
    type='SHIFTDataset',
    data_root='./SHIFT_dataset/discrete/images',
    img_prefix='train/front/img.zip',
    ann_file='train/front/det_3d.json',
    insseg_ann_file="train/front/det_insseg_2d.json",
    backend_type='zip',
    pipeline=[
        dict(type="LoadAnnotations3D", with_bbox=True, with_mask=True),
        ...
    ]
)