use nested folder

[waragai-katsunori]

why

• Curent version assumes images are in non-nested folders train2017 or val2017
• However nested folder is easier to know what sort of scenes are selected for training and evaluation.

what

• As far as COCO json file assumes non-nested folders, keep original img_file.
• If COCO json file assumes nested folders, use img_file for nested case.

Example

In case following folder structure,
keeping data/scene1 as traditional folder structure for COCO Json file format.
I have combined each json files with using nested folder structure
annotations/instances_train2017.json
annotations/instances_val2017.json

Now file_name is such like "file_name": "scene1/filename_1.jpg".

I wish this is useful.

data
data/annotations
data/scene1
data/scene1/annotations
data/scene1/train2017
data/scene1/val2017
data/scene2
data/scene2/annotations
data/scene2/train2017
data/scene2/val2017

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