code: def test_lstmseq2seqattn_forward_shape(self, backend):
      with fastmath.use_backend(backend):
          model = rnn.LSTMSeq2SeqAttn(
              input_vocab_size=20, target_vocab_size=20, d_model=16
          )
          x = np.ones((3, 28)).astype(np.int32)
          _, _ = model.init([shapes.signature(x), shapes.signature(x)])
          ys = model([x, x])
          self.assertEqual(
              [y.shape for y in ys], [(3, 28, 20), (3, 28)]
          )
  
file_path: trax/trax/models/rnn_test.py
imports: from absl.testing import absltest
  from absl.testing import parameterized
  import numpy
  from trax import fastmath
  from trax import shapes
  from trax.models import rnn
label_desc: Use image node_id 3 for calling the RNNTest obj's underlying member method code with example usage: obj.test_lstmseq2seqattn_forward_shape(backend) without return types