[DataPipe] Fix deepcopy filehandle for Mapper and in-place modification for IterableWrapper (#65220)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65220
Fixes #65221
- Remove deepcopy from Mapper to support file handles
- Convert `IterableWrapper` to deepcopy iterable instance within each iterator to prevent in-place modification (different data per epoch)
- Convert `IDP` to `IterableWrapper` in test_datapipe.py
- Refine the variable names (prevent using `dp` that is module reference)
Test Plan: Imported from OSS
Reviewed By: malfet
Differential Revision: D31021886
Pulled By: ejguan
fbshipit-source-id: 72a9eee66c758e2717d591cd0942892bddedc223
