ZeroDivisionError: centered_arange step size is 0

In `upsample.py` line 352, the step size (3rd param) passed to `np.arange` is 0, yielding a `ZeroDivisionError`. This is caused by `t` having value 1, and `reduction` having value 2: `1 // 2 = 0`. The fieldmap is `((0,0), (4,4), (1,1))`.

The conv layer that is being analyzed has a 4x4 kernel, 4 output channels, and a stride of 1. Its input size is 3x120x120, the output size is 4x117x117.

I am going to debug this further but just wanted to share this already.