message: Update CrossReplicaOptimizer  to use cross_replica_mean
  Summary:
  REF T47306
  Depends on D53430
  TF2.4 Only
  Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, georgep, samuelh, babakk, markf
  Maniphest Tasks: T47306
diff: @@ -28,8 +28,7 @@ def apply_cross_replica_op_single(grad, var):
  if grad is None:
  return (grad, var)
  with ops.colocate_with(grad):
  - return (gen_poputil_ops.ipu_replication_normalise(
  - cross_replica_ops.cross_replica_sum(grad)), var)
  + return (cross_replica_ops.cross_replica_mean(grad), var)
  def apply_cross_replica_op(grads_and_vars):
  