Fix mypy for IterDataPipe.collate (#81275)
Add `default_collate` to mypy stub file to make sure `default_collate` is imported for `IterDataPipe.collate`
Sister PR from TorchData: https://github.com/pytorch/data/pull/645
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81275
Approved by: https://github.com/NivekT
