Add DOTA dataset

[nilsleh]

This PR adds the DOTA dataset for object detection.

Dataset rehosted on HF for faster and comprehensive download in one place.

Dataset features:

• multi-class object detection (15 classes in V1 and 18 classes in V2)
• horizontal and oriented bounding boxes

Dataset format:

• images are three channel PNGs with various pixel sizes
• annotations are text files with one line per bounding box

Horizontal BBox example:

Oriented BBox example:

@dingjiansw101 as inquired in CAPTAIN-WHU/DOTA#29, this is the PR that aims to make the DOTA datset more accessible. The dataset structure in the google and baidu drives was a bit confusing and I restructured it on HF which is hopefully more clear. If you would take a look and have any comments, that would be helpful.

Also @robmarkcole perhaps interesting for you.

Closes #1