message: [XLA:Python] Add default arguments to some collective ops.
  Update some tests to use XlaBuilder instead of the ComputationBuilder wrapper.
diff: @@ -324,12 +324,13 @@ void BuildOpsSubmodule(py::module* m) {
  XlaOp, const XlaComputation&, absl::Span<const ReplicaGroup>,
  const absl::optional<ChannelHandle>&, const absl::optional<Shape>&)>(
  &AllReduce),
  - py::arg("operand"), py::arg("computation"), py::arg("replica_groups"),
  + py::arg("operand"), py::arg("computation"),
  + py::arg("replica_groups") = py::list(),
  py::arg("channel_id") = absl::nullopt,
  py::arg("shape_with_layout") = absl::nullopt);
  ops.def("AllToAll", &AllToAll, py::arg("operand"), py::arg("split_dimension"),
  py::arg("concat_dimension"), py::arg("split_count"),
  - py::arg("replica_groups"));
  + py::arg("replica_groups") = py::list());
  ops.def("CollectivePermute", &CollectivePermute, py::arg("operand"),
  py::arg("source_target_pairs"));
  ops.def("CreateToken", &CreateToken, py::arg("builder"));
  