Diagonal iteration instead of naive

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Hello,

First of all thanks for your work, it is to my knowledge the first public attempt at defining a multidimensional while_loop.

If I understood your implementation, each cell of the grid is updated sequentially from left to right, then up to down. But as pointed by your illustration in the ReadMe, this process is parallelizable, as each cell on the diagonal orthogonal to the propagation can be updated independantly. Do you have any lead on performing this processing? I tried to work on this one but could not figure out a solution, Tensorflow really isn't easy to use in this case because the number of cell states to compute at a time is a function of h and w.

Thanks, Quentin