Fix Mac mps dataloader_num_workers > 1 causes RuntimeError: _share_filename_: only available on CPU (#38819)
* Update trainer.py: add multiprocessing_context for mps devices
* Fix multiprocessing context for MPS with workers
* Apply style fixes
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Co-authored-by: Marc Sun <57196510+SunMarc@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>