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 __init__(self, config: synchronizers_pb2.PSSynchronizer):
      self.target_device = (
          config.reduction_destination
          if config.reduction_destination
          else ""
      )
      self._local_replication = config.local_replication
      self._sync = config.sync
      self._staleness = config.staleness
  
      self._var_op_to_agg_grad = {}
      self._var_op_to_accum_apply_op = {}
      super().__init__()
  
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 1 to create a new PSSynchronizer object from inherited base classes: Synchronizer with example: obj = PSSynchronizer(config)