broadcast_shapes turns a 0 dimension into 1 #2015

broadcast_shapes([(0,), (1,)]) returns (1,). NumPy returns (0,). broadcast_shapes([(5, 1), (1, 0)]) returns (5, 1). NumPy returns (5, 0). broadcast_shapes([(1, 0), (0, 1)]) returns (1, 1). NumPy returns (0, 0).

The function follows NumPy, and wp.map uses this shape for the output array. When one size is 1, the result is max of the two sizes, so 0 loses to 1. broadcast_shapes([(3, 1, 4), (5, 4)]) is still (3, 5, 4).