[Cosmos3 OSS]Add more action datasets

[lzhang57]

Dataset support added:

1. Fractal (FractalLeRobotDataset class)
2. RoboMind Frank dual arm (through existing RoboMINDFrankaDataset class)
3. RoboMind UR (RoboMINDURDataset class)

Other changes:

1. Add corresponding stats files for the newly added datasets
2. Folder structure refactor (minor)

[mli0603]

checks are failing, but otherwise LGTM

[lfengad]

LGTM