code: def test_wrapper_can_call_backward_with_zero_length_sequences(self):
      lstm = LSTM(
          bidirectional=True,
          num_layers=3,
          input_size=3,
          hidden_size=7,
          batch_first=True,
      )
      encoder = PytorchSeq2SeqWrapper(lstm)
      input_tensor = torch.rand([5, 7, 3])
      mask = torch.ones(5, 7).bool()
      mask[0, 3:] = False
      mask[1, 4:] = False
      mask[2, 0:] = 0  # zero length False
      mask[3, 6:] = False
  
      output = encoder(input_tensor, mask)
  
      output.sum().backward()
  
imports: import numpy
  from numpy.testing import assert_almost_equal
  import pytest
  import torch
  from torch.nn import LSTM, GRU
  from torch.nn.utils.rnn import pack_padded_sequence, pad_packed_sequence
  from allennlp.common.checks import ConfigurationError
  from allennlp.common.testing import AllenNlpTestCase
  from allennlp.modules.seq2seq_encoders import PytorchSeq2SeqWrapper
  from allennlp.nn.util import sort_batch_by_length, get_lengths_from_binary_sequence_mask
label_desc: Use image node_id 9 for calling the TestPytorchSeq2SeqWrapper obj's underlying member method code with example usage: obj.test_wrapper_can_call_backward_with_zero_length_sequences() without return types