From what I understand, the diagonal here is not the traditional diagonal of a matrix (elements m[i, i]), but the other diagonal (elements m[i, j] such that i + j == m.shape[-1]).
i0 and i1 look like the axes where we take the indices i and j. So, for a 3D tensor `t`, `get_diagonal_subtensor_view(t, 0, 1)` would contain the elements `t[i, j, :]` so that `i + j == t.shape[0]`; `get_diagonal_subtensor_view(t, 2, 0)` would contain the elements `t[j, :, i]` so that `i + j == t.shape[2]`.
Am I on the right track?
It looks like I'm missing something, because the unit tests seem to return more than one row around the diagonal.