Convert `notebooks/compute_stats_from_batches.ipynb` into a fully-automated script

[JackKelly]

The notebook created in PR #506 works. But it needs some manual intervention (e.g. to handle the slightly different data layout in batches for different modalities). This issue is about converting the notebook into a fully-automated script that can be run after we create new batches to compute new means and stds.

Related issue: #499