Refactor OpenX

[michel-aractingi]

What this does

Simplifies OpenX files by focusing only on the RLDS format. Remove unnecessary file from OpenVLA for training with OpenX. Now all RLDS datasets (OpenX and others) can use this format freely.

Adds missing features and observations from the RLDS format to LeRobotDataset, e.g., is_first, is_terminal, discount...

Example:

 python lerobot/scripts/push_dataset_to_hub.py \
        --raw-dir /fsx/michel_aractingi/datasets/maniskill_dataset_converted_externally_to_rlds/0.1.0 \
        --repo-id lerobot/maniskill_dataset \
        --raw-format rlds 

How it was tested

Visualize datasets to make sure they were pushed correctly

python lerobot/scripts/visualize_dataset_html.py --repo-id lerobot/maniskill_dataset --episodes 0 1 2 3 4

NOTE: will only push after LeRobotDatasetV2 is merged.