Distribution.from_shape is producing localarrays with size 0

[cowlicks]

With 4 engines:

In [1]: from distarray.dist import Context, Distribution

In [2]: context = Context()

In [3]: distribution = Distribution.from_shape(context, (9, 8), dist=('b', 'n')) 

In [4]: distribution.localshapes()
Out[4]: [(3, 8), (3, 8), (3, 8), (0, 8)]

This appears to be from this line.

The size of this axis of the grid_shape is set to comm_size, which doesn't divide into the size of the corresponding axis of the DistArray.

[cowlicks]

From some experimenting if we call the processor grid size for an axis p and the distarray size in an axis d then the following must be satisfied

p < d
d = p*n + p - n - 1

Where n = p // d so d - p < pn < d.

From experimenting, it looks like for a given p there are p - 2 cases where the above relations are satisfied and we would end up with an empty localarray.

It seems like the best solution would be to check if the above conditions are satified in make_grid_shape, if not reduce p by 1 and try again.

[kwmsmith]

Should be partially addressed by PR #502.