Fix the iterator category for torch::data::Iterator (#15500)
Summary:
Try to fix https://github.com/pytorch/pytorch/issues/14410.
Additional info: From this [page](https://stackoverflow.com/questions/14062297/canonical-way-to-define-forward-output-iterator), If we change it into `input_iterator_tag`, it doesn't mean the `output_iterator_tag` is lost.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15500
Differential Revision: D13545773
Pulled By: soumith
fbshipit-source-id: 327bfb7be83d53e42925e0e391b2a4277e3a1b36
