Got it, thanks 
for anyone wanting to do something similar, s reads in your samples, so this (in my case) was set as the following:
input image, bbox coord, bbox lbl, segmentation mask.
So I cared about 1:3 for the bounding box paddings
I then just did myBBoxBlock = TransformBlock(type_tfms=TensorBBox.create, item_tfms=PointScaler, dls_kwargs = {'before_batch': mybb_pad})