message: Rename distributed_apply to _distributed_apply in OptimizerV2 to match
  the Optimizer base class.
diff: @@ -866,7 +866,7 @@ class OptimizerV2(optimizer_v1.Optimizer):
  raise ValueError("No gradients provided for any variable: %s." %
  ([str(v) for _, v in grads_and_vars],))
  return distribute_lib.get_tower_context().merge_call(
  - self.distributed_apply, filtered, global_step=global_step, name=name)
  + self._distributed_apply, filtered, global_step=global_step, name=name)
  def _get_or_create_state(self, var_list=None):
  """Either looks up or creates `_OptimizerV2State`.
  @@ -899,7 +899,7 @@ class OptimizerV2(optimizer_v1.Optimizer):
  self._per_graph_state[graph_key] = per_graph_state
  return per_graph_state
  - def distributed_apply(self, distribution, grads_and_vars, global_step, name):
  + def _distributed_apply(self, distribution, grads_and_vars, global_step, name):
  """`apply_gradients` for use with a `DistributionStrategy`."""
  reduced_grads = distribution.batch_reduce("sum", grads_and_vars)
  var_list = [v for _, v in grads_and_vars]
  