message: Make collective ops work with only one replica.
diff: @@ -350,7 +350,7 @@ def build_collective_reduce(input_tensors,
  """
  group_size = len(input_tensors) * num_workers
  if group_size < 2:
  - raise ValueError('num_workers * len(input_tensors) must be 2 or greater')
  + return input_tensors
  devices = [t.device for t in input_tensors]
  num_devices = len(devices)
  group_key = collective_keys.get_group_key(devices)
  