enable coverage test for dataloader on Windows (#52550)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/50661
For coverage,
The class qualified name is `'SimpleCustomBatch': <class '__mp_main__.SimpleCustomBatch'>`
For pytest
The class qualified name is `'SimpleCustomBatch': <class 'test_dataloader.SimpleCustomBatch'>`
So move the class to one separate file
As malfet suggestion, use __import__ to avoid adding new file.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52550
Reviewed By: walterddr
Differential Revision: D26754023
Pulled By: malfet
fbshipit-source-id: 34b0fbe7336b9303cedc28ec6116ab752a2d3630
