code: def step_optimizer(ctx, model):
      if model.optimizer:
          model.optimizer.step()
  
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 4 for calling a global function with example usage: step_optimizer(ctx, model) without return types