A shape containing the union of all dimensions in the input shapes.

  Args:
    shapes: a list of Shapes

  Returns:
    a Shape

def _shape_union(shapes):
  """A shape containing the union of all dimensions in the input shapes.

  Args:
    shapes: a list of Shapes

  Returns:
    a Shape
  """
  return Shape(sorted(list(set(sum([s.dims for s in shapes], [])))))