Support for GPU-accelerated affine transformations

Affine transformations on CPU are already pretty fast, but GPU-accelerated transformations are useful in CPU-constrained environments like Google Colab where a GPU is available but only 2 mediocre CPUs which can lead to a bottleneck in the data pipeline.

Some open questions:

- Would the existing transforms work out-of-the box on `CuArray`s (i.e. does `ImageTransformations.warp[!]` work on `CuArray`s) ? What about for masks, i.e. integer arrays? Otherwise, what would be necessary to implement that?
- Performance-wise, do the transforms need to be applied to a whole batch at once or is it as fast to apply them to samples individually? Former would require first resizing images to the same size and wrapping in a `Batch` wrapper item.

Can someone who has experience with image transformations on GPU chime in?

@jsamaroo