code: def register_optimizer(ctx, model):
      if len(list(model.parameters())) > 0:
          model.optimizer = torch.optim.SGD(
              model.parameters(), lr=0.01, momentum=0.9
          )
      else:
          model.optimizer = None
  
file_path: fairscale/tests/nn/pipe_process/test_rpc.py
imports: import copy
  import os
  import pytest
  import torch
  from torch import nn
  from torch.distributed import rpc
  from fairscale.fair_dev.testing.testing import get_worker_map, torch_spawn
  from fairscale.internal import torch_version
  from fairscale.nn.model_parallel.initialize import get_pipeline_parallel_group
  from fairscale.nn.pipe import PipeRPCWrapper
label_desc: Use image node_id 3 for calling a global function with example usage: register_optimizer(ctx, model) without return types