Transform from v2d format into video_description format and save in video_description/ directory.

[kdu4108]

Goal: given v2d format of

 ├── 00000.tar
 |     ├── 00000.mp4
 |     ├── 00000.txt
 |     ├── 00000.json
 |     ├── 00001.mp4
 |     ├── 00001.txt
 |     ├── 00001.json
 |     └── ...
 |     ├── 10000.mp4
 |     ├── 10000.txt
 |     ├── 10000.json
 ├── 00001.tar
 |     ├── 10001.mp4
 |     ├── 10001.txt
 |     ├── 10001.json
 │     ...
 ...

produce a video_description/ modality data folder of the following format:

root/video_description/shard-00000.tar
 |     ├── 00000.jsonl # this corresponds to one video. each line within it corresponds to one subsequence of frames.
 |     ├── 00001.jsonl
 |     └── ...

Each jsonl should look something like

[
            {
                "description": "here's a description",
                "start_frame_index": 0,
                "end_frame_index": 5,
            },
            {
                "description": "here's another description",
                "start_frame_index": 5,
                "end_frame_index": 12,
            } 
]

Note that the txt/jsons in the v2d might not correspond exactly to the representation we want here (e.g., we might need some logic to determine the start/end frame indices from timestamps).

Where are these descriptions coming from? Do we pseudolabel them out with another description model? @smontariol?
Child issue of #3.