set_device_map(to, device_map)
  Set device mapping between each RPC caller and callee pair. This
  function can be called multiple times to incrementally add
  device placement configurations.

  Parameters:
     * **to** (*str*) -- Callee name.

     * **device_map** (*Dict of python:int**, **str**, or
       **torch.device*) -- Device placement mappings from this
       worker to the callee. This map must be invertible.

  -[ Example ]-

  >>> # both workers
  >>> def add(x, y):
  >>>     print(x)  # tensor([1., 1.], device='cuda:1')
  >>>     return x + y, (x + y).to(2)
  >>>
  >>> # on worker 0
  >>> options = TensorPipeRpcBackendOptions(
  >>>     num_worker_threads=8,
  >>>     device_maps={"worker1": {0: 1}}
  >>>     # maps worker0's cuda:0 to worker1's cuda:1
  >>> )
  >>> options.set_device_map("worker1", {1: 2})
  >>> # maps worker0's cuda:1 to worker1's cuda:2
  >>>
  >>> rpc.init_rpc(
