Multiprocessing Preprocessing Pickling Error

[mexicantexan]

Not fully tested, but by moving the functions/classes causing the error outside of their respective functions into a broader scope the pickling error went away.

• Move collate_fn function outside of dataset_to_dataloader function
• Move ImageDataset class outside of get_image_dataset function

Should address:

• AttributeError: Can't pickle local object 'get_image_dataset.<locals>.ImageDataset' #220
• issue with inference #345

With this change, was able to set the num_prepro_workers to > 0.

[mexicantexan]

Tests are broken, doesn't seem like the tar files are getting extracted after they are created by img2dataset. I'm on a Windows machine using Python 3.10.