docstr: Assert that the training loss and validation error decrease monotonically.
code: def test_onevar_single(self, tmp_path: pathlib.Path) -> None:
      """Assert that the training loss and validation error decrease monotonically."""
      tensorboard_path = tmp_path.joinpath("tensorboard")
      (
          trial,
          trial_controller,
      ) = pytorch_utils.create_trial_and_trial_controller(
          trial_class=pytorch_onevar_model.OneVarTrial,
          hparams=self.hparams,
          trial_seed=self.trial_seed,
          tensorboard_path=tensorboard_path,
      )
  
      train_steps, metrics = trial_controller._train_with_boundaries(
          training_enumerator=enumerate(
              trial_controller.training_iterator
          ),
          train_boundaries=[
              pytorch._TrainBoundary(
                  step_type=pytorch._TrainBoundaryType.TRAIN,
                  unit=pytorch.Batch(100),
              )
          ],
      )
  
      assert len(train_steps) == 1, "unexpected train step count"
      assert train_steps[
          0
      ].limit_reached, "train step did not reach expected limit"
      assert len(metrics) == 100, "metrics length did not match input"
  
      for i in range(100):
          pytorch_onevar_model.OneVarTrial.check_batch_metrics(
              metrics[i],
              i,
              metric_keyname_pairs=(
                  ("loss", "loss_exp"),
                  ("w_after", "w_exp"),
              ),
          )
      for older, newer in zip(metrics, metrics[1:]):
          assert newer["loss"] <= older["loss"]
  
imports: import contextlib
  import importlib
  import io
  import os
  import pathlib
  import sys
  import typing
  from unittest import mock
  import numpy
  import pytest
  import torch
  from _pytest import monkeypatch
  from torch.distributed import launcher
  import determined
  from determined import pytorch
  from tests.experiment import pytorch_utils, utils
  from tests.experiment.fixtures import pytorch_onevar_model
label_desc: Use image node_id 2 for calling the TestPyTorchTrial obj's underlying member method code with example usage: obj.test_onevar_single(tmp_path) without return types