add root argument to the dataset visualizer to visualize local datasets

[tlpss]

What this does

This PR adds the root argument to the dataset visualizer. This parameter is passed to LeRobotDataset.__init__, which enables visualizing local datasets, as an alternative to setting the DATA_DIR env variable.

closes #248

How it was tested

Explain/show how you tested your changes.

• added a unit test to tests/test_visualize_dataset that visualizes the local copy of the lerobot/pusht dataset

How to checkout & try? (for the reviewer)

Examples:

python lerobot/scripts/visualize_dataset.py --repo-id lerobot/pusht --root tests/data/ --episode-index 0 --mode distant

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This change is 

[tlpss]

@Cadene feel free to provide feedback!