Vision: `aligned` flag in ROIAlign

[zsef123]

In Detectron2, they got the alignd flag in ROIAlign.

It's about pixel shift, More details in docstring

Are you planning on going forward about this flag?
Or can I PR?

[fmassa]

I think adding a aligned flag to RoIAlign makes sense and would be a good contribution.

Also, ccing @ppwwyyxx and @rbgirshick as they might be interested in having those available in torchvision

[rbgirshick]

I think it would be great to add this feature to torchvision. Then I think we should be able to switch to using it in D2 instead of a custom implementation.