Make pin_memory and default_collate preserve namedtuples (#16440)
Summary:
Open issue: https://github.com/pytorch/pytorch/issues/3281
Corresponding PR (conflict): https://github.com/pytorch/pytorch/pull/4577
Another open issue: https://github.com/pytorch/pytorch/issues/14613
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16440
Differential Revision: D14020901
Pulled By: ezyang
fbshipit-source-id: 4abe817fc43c281a510715d311bad544511995d3
