code: def _prepareTrials(self, scheduler, base_pgf):
      trial1 = Trial("mock", config=dict(num=1), stub=True)
      trial1.placement_group_factory = base_pgf
      trial2 = Trial("mock", config=dict(num=2), stub=True)
      trial2.placement_group_factory = base_pgf
      trial3 = Trial("mock", config=dict(num=3), stub=True)
      trial3.placement_group_factory = base_pgf
      trial4 = Trial("mock", config=dict(num=4), stub=True)
      trial4.placement_group_factory = base_pgf
  
      self.tune_controller._trials = [trial1, trial2, trial3, trial4]
  
      scheduler.on_trial_add(self.tune_controller, trial1)
      scheduler.on_trial_add(self.tune_controller, trial2)
      scheduler.on_trial_add(self.tune_controller, trial3)
      scheduler.on_trial_add(self.tune_controller, trial4)
  
      trial1.status = Trial.RUNNING
      trial2.status = Trial.RUNNING
      trial3.status = Trial.RUNNING
      trial4.status = Trial.RUNNING
      return trial1, trial2, trial3, trial4
  
imports: import shutil
  import tempfile
  import unittest
  from ray.tune import PlacementGroupFactory
  from ray.tune.execution.tune_controller import TuneController
  from ray.tune.schedulers.trial_scheduler import TrialScheduler
  from ray.tune.experiment import Trial
  from ray.tune.schedulers.resource_changing_scheduler import ResourceChangingScheduler, DistributeResources, DistributeResourcesToTopJob
  from ray.tune.tests.execution.utils import create_execution_test_objects
  from ray.train.tests.util import mock_storage_context
label_desc: Use image node_id 3 for calling the TestUniformResourceAllocation obj's underlying member method code with example usage: obj._prepareTrials(scheduler, base_pgf) and returns: trial1,  trial2,  trial3,  trial4