def test_fixed_seq_len(self):
        """
        Input has a fixed sequence length.
        (this happens when model is trained using padded sequences, inspiration: https://forums.developer.apple.com/thread/80407)

        (Seq,Batch,C,H,W)
        embedding: input shape (15,1,1,1,1) --> output shape (15,1,32,1,1)
        permute  : input shape (15,1,32,1,1) --> output shape (1,1,32,1,15)
        flatten  : input shape (1,1,32,1,15) --> output shape (1,1,32 * 15,1,1)
        dense    : input shape (1,1,480,1,1) --> output shape (1,1,2,1,1)
        """

        coreml_preds = []
        input_dim = (1, 1, 1)
        output_dim = (
            1,
            1,
            1,
        )  # some random dimensions here: we are going to remove this information later
        input_features = [("data", datatypes.Array(*input_dim))]
        output_features = [("output", datatypes.Array(*output_dim))]
        builder = neural_network.NeuralNetworkBuilder(input_features, output_features)

        # ADD Layers
        builder.add_embedding(
            "embed",
            W=np.random.rand(10, 32),
            b=None,
            input_dim=10,
            output_channels=32,
            has_bias=0,
            input_name="data",
            output_name="embed",
        )
        builder.add_permute(
            "permute", dim=[3, 1, 2, 0], input_name="embed", output_name="permute"
        )
        builder.add_flatten(
            "flatten", mode=0, input_name="permute", output_name="flatten"
        )
        builder.add_inner_product(
            "dense",
            W=np.random.rand(480, 2),
            b=None,
            input_channels=480,
            output_channels=2,
            has_bias=0,
            input_name="flatten",
            output_name="output",
        )

        # Remove output shape by deleting and adding an output
        del builder.spec.description.output[-1]
        output = builder.spec.description.output.add()
        output.name = "output"
        output.type.multiArrayType.dataType = coremltools.proto.FeatureTypes_pb2.ArrayFeatureType.ArrayDataType.Value(
            "DOUBLE"
        )

        # save the model
        model_dir = tempfile.TemporaryDirectory()
        model_path = os.path.join(model_dir.name, "test_layer.mlmodel")
        coremltools.utils.save_spec(builder.spec, model_path)
        # prepare input and get predictions
        coreml_model = coremltools.models.MLModel(model_path)
        X = np.random.randint(low=0, high=10, size=15)
        X = np.reshape(X, (15, 1, 1, 1, 1)).astype(np.float32)
        coreml_input = {"data": X}
        if _is_macos() and _macos_version() >= (10, 13):
            coreml_preds = coreml_model.predict(coreml_input)["output"]
            self.assertEqual(len(coreml_preds.flatten()), 2)

        Input has a fixed sequence length.
        (this happens when model is trained using padded sequences, inspiration: https://forums.developer.apple.com/thread/80407)

        (Seq,Batch,C,H,W)
        embedding: input shape (15,1,1,1,1) --> output shape (15,1,32,1,1)
        permute  : input shape (15,1,32,1,1) --> output shape (1,1,32,1,15)
        flatten  : input shape (1,1,32,1,15) --> output shape (1,1,32 * 15,1,1)
        dense    : input shape (1,1,480,1,1) --> output shape (1,1,2,1,1)
        
coremltools/test/neural_network/test_custom_neural_nets.py