ENH: Implement pt.pad

[jessegrabowski]

Description

Numpy, along with derivative tensor libraries like jax, torch, etc, all offer padding functions to insert values to the left and right (and top and bottom, etc) to a tensor. We should probably have this as well. It shouldn't be too difficult (famous last words) -- I see it essentially as some shape computation followed by an AllocSubtensor, although we could certainly try to figure out the best way to do this.