Clean up prototype transforms before migration

The transforms in torchvision.transforms are fully JIT scriptable. To achieve this we needed a lot of helper methods and unnecessary strict or plain wrong annotations.

The revamped transforms in torchvision.prototype.transforms will no longer be JIT scriptable due to their ability to handle more complex inputs. Thus, we should clean-up our code removing everything that was just added to appease torch.jit.script. This should only happen as one of the last steps before migration to the main area.

cc @vfdev-5 @datumbox @bjuncek

[datumbox]

I would go with option 3. IMO the current typing annotations provide sufficient info without introducing complex solutions like 2.

[pmeier]

Regardless of #6584, the transforms will not be JIT scriptable. Thus, cleaning them up is still relevant.