Change len(DataLoader) for IterableDataset (#38925)
Summary:
Fix https://github.com/pytorch/pytorch/issues/36176
One-liner change to ensure that ```len(loader) == (len(dataset) // batch_size)``` for IterableDataset.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38925
Differential Revision: D21731587
Pulled By: ezyang
fbshipit-source-id: 59a086165a004c0c1c8a1ee0776b1444bd26de23
