message: Support specifying sharding attribute for tensors with unknown dimension
  There is nothing in the sharding API what would require a known
  dimension so there is no reason to require it.
diff: @@ -104,9 +104,9 @@ class Sharding(object):
  ValueError: The tensor to split was smaller in the split dimension than
  the number of devices to split over.
  """
  - tensor.shape.assert_is_fully_defined()
  shape = tensor.shape.as_list()
  - if shape[split_dimension] < num_devices:
  + if (shape[split_dimension] is not None and
  + shape[split_dimension] < num_devices):
  raise ValueError('Split dimension was smaller than the required number '
  'of splits: shape=%r, dimension=%r, num_devices=%r' %
  (shape, split_dimension, num_devices))
  