Add train/val/test split script for raw

[kdu4108]

Given a folder data/raw/... where ... = the downloaded v2d datasets, we need a script to partition (move) them into subdirectories called train, val, and test.

This is because the 4M folder directories expect the train/val/test to precede the modality, e.g.

train/video_rgb/class0/000.tar
train/video_det/class0/000.tar
train/video_transcript/class0/000.tar
```.

Then we can just run the merge_data.sh to go from raw to 4m-data 3 times (one for each split).

For now we can use a split of 70/10/20.

[markus583]

See #15