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.

Start in upsample.py at line 352 and trace centered_arange with the reported fieldmap, kernel, stride, and input/output sizes. Reproduce the t=1 and reduction=2 case, then determine the expected behavior when the computed np.arange step is zero. Done means the case no longer raises ZeroDivisionError and the relevant behavior is covered by an appropriate test.

Written by the indexing model from the issue text.