[Data] Refine the skeleton extraction script

[kennymckormick]

1. Support non-distributed skeleton extraction by adding a --non-dist flag. With that flag, you can directly run the skeleton extraction python script: python tools/data/custom_2d_skeleton.py {args} --non-dist
2. Add a new option to generate compressed skeleton annotations (the K400 style). This can be achieved by set --compress and you will have a new filed name frame_inds in your generated annotations, indicates the frame index that each skeleton belongs to.