code: def test_decoder_forward_backward(self):
      for num_flow_blocks_dec in [8, None]:
          for hidden_channels_dec in [100, None]:
              decoder = self._get_decoder(
                  num_flow_blocks_dec, hidden_channels_dec
              )
              _, _, mel_spec, mel_lengths = _create_inputs()
              z, z_len, _ = decoder(
                  mel_spec.transpose(1, 2), mel_lengths
              )
              mel_spec_, mel_lengths_, _ = decoder(
                  z, z_len, reverse=True
              )
              mask = sequence_mask(z_len).unsqueeze(1)
              mel_spec = (
                  mel_spec[:, : z.shape[2], :].transpose(1, 2) * mask
              )
              z = z * mask
              self.assertTrue(
                  torch.isclose(mel_spec, mel_spec_, atol=1e-2).all(),
                  f"num_flow_blocks_dec={num_flow_blocks_dec}, hidden_channels_dec={hidden_channels_dec}",
              )
  
imports: import os
  import random
  import unittest
  from copy import deepcopy
  import torch
  from tests import get_tests_output_path
  from TTS.tts.configs.overflow_config import OverflowConfig
  from TTS.tts.layers.overflow.common_layers import Encoder, Outputnet, OverflowUtils
  from TTS.tts.layers.overflow.decoder import Decoder
  from TTS.tts.layers.overflow.neural_hmm import EmissionModel, NeuralHMM, TransitionModel
  from TTS.tts.models.overflow import Overflow
  from TTS.tts.utils.helpers import sequence_mask
  from TTS.utils.audio import AudioProcessor
label_desc: Use image node_id 2 for calling the TestOverflowDecoder obj's underlying member method code with example usage: obj.test_decoder_forward_backward() without return types