Set up COCO keypoints for annotation

[garyfeng]

We will use COCO Annotator for keypoint annotation. It requires setting up a category for annotation. In my playing with it, I set up a category called swimmer, which had a subset of keypoints defined -- "neck", "shoulder-left", etc. I could then annotate this category in the datasets.

For this actual annotation, we need to:

1. Formally set up a category = swimmer
2. Define the keypoints following precisely the COCO datasets: see https://github.com/facebookresearch/Detectron/issues/640
3. Test the set up end-to-end:
   • Category and annotation
   • Setting up the library and the metadata
   • Exporting the COCO annotation data for training
   • Actually carrying out a training session, with say AlphaPose or PaddleDetect
4. Set up the actual COCO Annotator project with this category

[garyfeng]

see https://github.com/garyfeng/swimpose/issues/16