Add latest version of torchvision to requirements and eliminate copy-pasted code

[rstojnic]

We should add latest pytorch/torchvision installation from github into requirements, then eliminate all copy-pasted code.

[RJT1990]

Update: this has been streamlined now, but I still need some customisation:

• Cityscapes : I need auto extraction of zips (not in torchvision github), and also access to dual transforms logic (in torchvision github, not in stable release). Have made a PR to torchvision.

• COCO - am using an inherited cocodetection class (following the example in vision/references). I will need to add autoextraction logic to this too, so will probably send over another pull request.

Are we happy btw to add an unstable release of torchvision as a requirement for our users? Just worried about unfinalized API changes etc.