Add COCOBboxDataset

[yuyu2172]

Depends on #452

I think we need some modification to the BboxDataset API.

1. bbox can be zero-sized (0,4).
2. Make it easier to filter images with no ground truth bounding boxes. I suggest to make a public method labels for all bbox datasets. The method returns a list of bounding boxes. Also, I suggest to support similar method for all label datasets.

[Hakuyume]

Make it easier to filter images with no ground truth bounding boxes. I suggest to make a public method labels for all bbox datasets. The method returns a list of bounding boxes. Also, I suggest to support similar method for all label datasets.

I guess the advantage of labels is that users can filter images without loading them. Is ii right?

[yuyu2172]

Yea

[Hakuyume]

I made a mix-in for this problem. Please check #454.

[yuyu2172]

Areas of instance masks are needed for evaluation. So, I added an option that makes the dataset to return them.

I've been working on evaluation of COCO as well. My work can be found here.
https://github.com/yuyu2172/coco-evaluation/tree/master/detection

[yuyu2172]

@Hakuyume

[Hakuyume]

@yuyu2172 Could you merge master branch?

[yuyu2172]

ok

[Hakuyume]

5,000 images?

[Hakuyume]

I prefer MS COCO because we use PASCAL VOC in this doc.

[Hakuyume]

LGTM