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* Replace type T with accumulator.

* Upcast tensors of box ops to avoid overflow in multiplications.

Co-authored-by: Vasilis Vryniotis <datumbox@users.noreply.github.com>

* Adding TORCH_SELECTIVE_* macros on op registration.

* Adding torchvision namespace.

Summary:
* Reduce unnecessary header inclusions in models and io.

* Move autocast to separate folder and hide autograd implementation in an anonymous namespace.

* Moving files in subfolders.

Reviewed By: fmassa

Differential Revision: D25461523

fbshipit-source-id: 756eeb6848aacaa474de4825ed4c1045d17e2cea