outer product $\otimes$

Computes

$$z_{i_1,i_2,\ldots,i_{r_x},j_1,\ldots,j_{r_y}} = x_{i_1,,i_2,\ldots,i_{r_x}}y_{j_1,\ldots,j_{r_y}}.$$

where $r_x$ and $r_y$ are the ranks of $x$ and $y$ respectively.