class_docstr: ReplicaContext for strategy that is backed by DTensor.
  
  Since the DTensor is operated in the global context, most of the methods from
  existing strategy ReplicaContext is not applicable since they need to access
  local values. For now most of the methods in this class will raise explicit
  error to user, and we will add more support for local values in future.
code: def all_reduce(self, reduce_op, value, options=None):
      raise NotImplementedError(self._UNSUPPORTED_ERROR_MSG)
  
imports: from tensorflow.dtensor.python import accelerator_util
  from tensorflow.dtensor.python import api
  from tensorflow.dtensor.python import input_util
  from tensorflow.dtensor.python import layout
  from tensorflow.python.distribute import cross_device_ops
  from tensorflow.python.distribute import device_util
  from tensorflow.python.distribute import distribute_lib
  from tensorflow.python.distribute import reduce_util
  from tensorflow.python.distribute import values
  from tensorflow.python.eager import context
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import tensor_conversion_registry
  from tensorflow.python.framework import tensor_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import summary_ops_v2
label_desc: Use image node_id 6 for calling the DTensorReplicaContext obj's underlying member method code with example usage: obj.all_reduce(reduce_op, value, options) without return types