message: Improve error message for StructuredTensor gather when a negative axis can't be converted to a positive axis because the rank of `params` is unknown.
diff: @@ -127,8 +127,8 @@ def gather(params,
  with ops.name_scope(name):
  if axis is None:
  axis = batch_dims
  - ndims_name = params.shape.rank
  - axis = array_ops.get_positive_axis(axis, ndims_name)
  + axis = array_ops.get_positive_axis(axis, params.shape.rank,
  + ndims_name='params.shape.rank')
  indices = ragged_tensor.convert_to_tensor_or_ragged_tensor(
  indices, name='indices')
  