class_docstr: PS Synchronizer.
  
  Synchronizes gradient updates using a Parameter Server.
  
  For in-graph synchronization, this just aggregates gradients on a worker's CPU.
  
  For between-graph synchronization, this aggregates gradients on a pre-defined
  (defined in the Strategy) parameter server.
  
  To keep this gradient aggregation in sync, the chief gives each worker a token
  for each variable for the workers to mark when their variable update is complete.
code: def _get_aggregated_dense_grad(
      self, graph_item, grad_name, reduce_to_device
  ):
      grad_op_name = strip_replica_prefix(get_op_name(grad_name))
      output_idx = get_index_from_tensor_name(grad_name)
      grad_ops = [
          graph_item.graph.get_operation_by_name(
              ops.prepend_name_scope(grad_op_name, replica_prefix(i))
          )
          for i in range(self.num_replicas)
      ]
  
      # Aggregate gradients on `reduce_to_device` (usually CPU)
      with ops.device(reduce_to_device):
          grad_sum_op_name = ops.prepend_name_scope(
              grad_op_name, "%sAdd" % AUTODIST_PREFIX
          )
          grad_sum = math_ops.add_n(
              [grad_op.outputs[output_idx] for grad_op in grad_ops],
              name=grad_sum_op_name,
          )
          grad_avg_op_name = ops.prepend_name_scope(
              grad_op_name, "%sDiv" % AUTODIST_PREFIX
          )
          grad_avg = math_ops.realdiv(
              grad_sum, self.num_replicas, name=grad_avg_op_name
          )
      return grad_avg
  
imports: from functools import partial
  from typing import List
  from tensorflow.python import ops
  from tensorflow.python.framework import device_spec, dtypes, constant_op
  from tensorflow.python.platform import tf_logging
  from tensorflow.python.ops import math_ops, data_flow_ops, gen_control_flow_ops, control_flow_ops, gen_math_ops, gen_array_ops
  from autodist.const import MAX_INT64, AUTODIST_PREFIX
  from autodist.kernel.common import utils, variable_utils
  from autodist.kernel.common.op_info import UPDATE_OP_VAR_POS
  from autodist.kernel.common.proxy_variable import ProxyVariable
  from autodist.kernel.common.utils import get_op_name, get_consumers, get_ancestors, traverse, update_consumers, update_control_consumers, replica_prefix, strip_replica_prefix, get_control_consumers, remove_from_control_consumers, get_index_from_tensor_name, update_colocation_group
  from autodist.kernel.common.variable_utils import get_read_var_ops
  from autodist.kernel.synchronization.synchronizer import Synchronizer
  from autodist.proto import synchronizers_pb2
label_desc: Use image node_id 11 for calling the PSSynchronizer obj's underlying member method code with example usage: obj._get_aggregated_dense_grad(graph_item, grad_name, reduce_to_device) and returns: grad_avg