Extract sample data loaders for dependency reasons

[mathemancer]

Fixes #3367

This extracts the load_library_dataset and load_movies_dataset functions to their own modules, so these functions aren't dependent on the arxiv library being available (since they don't actually use it)

Technical details

Previously, these functions were in a larger module that also included a sample ArXiV dataset, which did need the arxiv module to work.

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[Anish9901]

Looks good to me @mathemancer, Thanks!