code: def test_multi_trainer():
      x = gluon.Parameter("x", shape=(10,), stype="row_sparse")
      x.initialize()
      # test set trainer
      trainer0 = gluon.Trainer([x], "sgd")
      assert x._trainer() is trainer0
      # test unset trainer
      x._set_trainer(None)
      assert x._trainer is None
      x._set_trainer(trainer0)
      with pytest.raises(RuntimeError):
          # multiple trainers for a sparse Parameter is not allowed
          trainer1 = gluon.Trainer([x], "sgd")
  
imports: import mxnet
  import unittest
  import os
  import numpy
  from mxnet import gluon
  from mxnet.gluon import nn
  from mxnet.test_utils import assert_almost_equal
  from common import assertRaises, xfail_when_nonstandard_decimal_separator
  from copy import deepcopy
  import pytest
label_desc: Use image node_id 2 for calling a global function with example usage: test_multi_trainer() without return types