Refactor dataloader.py (#15331)

Refactor dataloader.py (#15331)

Summary:
Same as #14668, and was approved there.

ailzhang , please apply this patch to Horizon's `data_streamer.py`: https://gist.github.com/SsnL/020fdb3d6b7016d81b6ba1d04cc41459 Thank you!

Below is the original description at #14668:

As I am working on tasks in https://github.com/pytorch/pytorch/issues/13023, I realized how unreadable the code is because all functions to be run in multiprocessing must be at top global level. Adding more functionalities to `dataloader.py` will only make things worse.

So in this PR, I refactor `dataloader.py` and move much of it into `data._utils`. E.g., the `_worker_loop` and related methods are now in `data._utils.worker`, signal handling code in `data._utils.signal_handling`, collating code in `data._utils.collate`, etc. This split, IMHO, makes code much clearer. I will base my future changes to DataLoader on top of this.

No functionality is changed, except that  I added `torch._six.queue`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15331

Reviewed By: yf225

Differential Revision: D13503120

Pulled By: ailzhang

fbshipit-source-id: 94df16b4d80ad1102c437cde0d5a2e62cffe1f8e