Add script to generate embedding for dataset

[pkooij]

This PR introduces a way to generate image and text embeddings to a dataset to be more efficient when training a dataset for multiple epochs. For example for learning a general reward we contain a specific dataset with OXE to improve generalization. In order to nor recompute the image and text embeddings each time we finetune for OXE we can use this script to add the embeddings to the dataset. We can additionally remove the videos in the dataset to safe space.

Testing:
Both the generate and validate script were tested on this dataset: lerobot/utokyo_xarm_bimanual. The generated dataset can be found here: pepijn223/utokyo_xarm_bimanual_embeddings.