code: def test_pop_unknown():
      @skippable(pop=["foo"])
      class Layer1(nn.Module):
          pass
  
      with pytest.raises(TypeError) as e:
          verify_skippables(nn.Sequential(Layer1()))
      assert (
          "'0' declared 'foo' as poppable but it was not stashed"
          in str(e.value)
      )
  
file_path: fairscale/tests/nn/pipe/skip/test_verify_skippables.py
imports: import pytest
  from torch import nn
  from fairscale.nn.pipe.skip import Namespace, skippable, verify_skippables
label_desc: Use image node_id 3 for calling a global function with example usage: test_pop_unknown() without return types