docstr: Runs a training loop, asserting that callbacks are initialized.
docstr_tok: ["Runs","a","training","loop",",","asserting","that","callbacks","are","initialized","."]
code: def test_initializes_step_callbacks_with_loop_instance(self):
      """Runs a training loop, asserting that callbacks are initialized."""
  
      class ActualLoop:
          # Wrapper object to make the Loop reference mutable.
          loop = None
  
      class TestCallback(callbacks.TrainingStepCallback):
          def __init__(self, loop):
              super().__init__(loop)
              ActualLoop.loop = loop
  
          def call_at(self, step):
              return False
  
          def on_step_begin(self, step):
              del step
  
          def on_step_end(self, step):
              del step
  
      model = tl.Serial(tl.Dense(1))
      task = training.TrainTask(
          _very_simple_data(), tl.L2Loss(), optimizers.SGD(0.01)
      )
      expected_loop = training.Loop(
          model, [task], callbacks=[TestCallback]
      )
      self.assertIs(ActualLoop.loop, expected_loop)
  
imports: import collections
  import os
  import time
  from absl.testing import absltest
  from jax import config
  import numpy
  from trax import data
  from trax import fastmath
  from trax import layers
  from trax import optimizers
  from trax import shapes
  from trax import test_utils
  from trax.layers import base
  from trax.models import transformer
  from trax.supervised import callbacks
  from trax.supervised import training
label_desc: Use image node_id 25 for calling the TrainingTest obj's underlying member method code with example usage: obj.test_initializes_step_callbacks_with_loop_instance() and returns: False