Ok, here's a docstring that should (I hope) explain what's going with the two related Ops in this file.

X is some n-dimensional tensor, but this Op only deals with a matrix-shaped slice, using axes `i0` and `i1`. Without loss of generality, suppose that 'i0' picks out our "row" dimension, and `i1` the "column" dimension.

So the relevant part of `x` is some matrix `u`. Suppose it has 7 rows and 4 columns

```
[ 0 0 0 0 ]
[ 0 0 0 0 ]
[ 0 0 0 0 ]
[ 0 0 0 0 ]
[ 0 0 0 0 ]
[ 0 0 0 0 ]
```

The view returned by this function is also a matrix. It's a thick, diagonal "stripe" across `u` that discards the lower left triangle and the upper right triangle:

```
[ x 0 0 0 ]
[ x x 0 0 ]
[ x x x 0 ]
[ 0 x x x ]
[ 0 0 x x ]
[ 0 0 0 x ]
```
In this case the return value would be this view of shape 3x4. The returned view has the same number of dimensions as the input `x`, and the only difference is that the shape along dimension `i0` has been reduced by `shape[i1] - 1` because of the triangles that got chopped out.

The `NotImplementedError` is meant to catch the case where shape[i0] is too small for the stripe to reach across the matrix, in which case it's not clear what this function should do. Maybe always raise an error. I'd look back to the call site in the Conv3D to see what's necessary at that point.