Bug in dataloader iterator found by mypy

[suo]

While upgrading mypy, found a call to _BaseDataLoaderIter.reset() that appears to trivially fail (link), as it does not provide a required positional argument.

cc @ssnl @VitalyFedyunin @ejguan @NivekT

[158ice]

FileNotFoundError: [Errno 2] No such file or directory: './data/train/NORMAL/img_7_44.jpg'