Copy paste prototype.datapoints and prototype.transforms out of prototype area

[NicolasHug]

This PR is against the migration_transforms_v2 branch (not main!), which we'll use to perform the migration and cleanups. When ready, we'll merge it with main.

This is an initial brutal copy paste:

• torchvision.prototype.datapoints -> torchvision.datapoints
• torchvision.prototype.transforms -> torchvision.transforms.v2

There will be a bunch of follow-up cleanups

CC @vfdev-5 @pmeier

[NicolasHug]

Since this is not against main I'll merge and we'll iterate on it with @pmeier and @vfdev-5