Fix: item transform dataset argument

[srishtiii28]

Closes #399

R CMD check flagged 18 unlockBinding(".getitem", ...) calls, used by the item_transform_*() and target_transform_rotate() dataset methods to patch a dataset's .getitem() in place.

Detection and segmentation datasets now take an item_transform argument,applied to the item after transform and
target_transform. Passing a dataset to item_transform_*() or target_transform_rotate() composes onto that argument instead of unlocking the binding, so the pipe form still works.

[cregouby]

Hello @srishtiii28,

This is a great addition, thank you so much.