SliceFilter is an optional argument but is required

[olix86]

slice_filter_fn is an optional argument but it's called whether it is defined or not. It should be either required or optional but called only if defined.
This would avoid the rather cryptic TypeError: 'NoneType' object is not callable

medicaltorch/medicaltorch/datasets.py

Line 237 in 3887d68

transform=None, slice_filter_fn=None, canonical=False):

medicaltorch/medicaltorch/datasets.py

Line 261 in 3887d68

filter_fn_ret_seg = self.slice_filter_fn(slice_seg_pair)