[ONNX] Support SequenceEmpty op (#13866)

* add SequenceEmpty

* add SequenceEmpty test

* pylint fix

---------

Co-authored-by: Valery Chernov <[email protected]>

Author: vvchernov

python/tvm/relay/frontend/onnx.py

@@ -6148,6 +6148,15 @@ def _impl_v11(cls, inputs, attr, params):
         return _expr.Tuple(inputs)
 
 
+class SequenceEmpty(OnnxOpConverter):
+    """Operator converter for sequence empty op."""
+
+    @classmethod
+    def _impl_v11(cls, inputs, attr, params):
+        # Construct an empty tuple.
+        return _expr.Tuple([])
+
+
 class SequenceErase(OnnxOpConverter):
     """Operator converter for sequence erase op."""
 
@@ -6523,6 +6532,7 @@ def _get_convert_map(opset):
         "LinearRegressor": LinearRegressor.get_converter(opset),
         # Sequence operators
         "SequenceConstruct": SequenceConstruct.get_converter(opset),
+        "SequenceEmpty": SequenceEmpty.get_converter(opset),
         "SequenceErase": SequenceErase.get_converter(opset),
         "SequenceInsert": SequenceInsert.get_converter(opset),
         "SequenceLength": SequenceLength.get_converter(opset),

tests/python/frontend/onnx/test_forward.py

@@ -7829,6 +7829,38 @@ def verify_sequence_ops(tensor_shape, num_tensors, axis=0, position=0, new_axis=
     verify_sequence_ops((3, 3, 3, 3), 4, axis=2, new_axis=1)
 
 
+@tvm.testing.parametrize_targets
+def test_empty_sequence(target, dev):
+    """test_empty_sequence"""
+
+    # Test creating an empty tensor sequence.
+    empty_node = helper.make_node(
+        "SequenceEmpty",
+        inputs=[],
+        outputs=["empty_sequence"],
+    )
+
+    length_node = helper.make_node("SequenceLength", inputs=["empty_sequence"], outputs=["output"])
+
+    graph_outputs = [helper.make_tensor_value_info("output", TensorProto.INT64, [])]
+
+    graph_nodes = [empty_node, length_node]
+
+    graph = helper.make_graph(
+        graph_nodes,
+        "Sequence_empty_test",
+        inputs=[],
+        outputs=graph_outputs,
+    )
+
+    model = helper.make_model(
+        graph,
+        producer_name="Sequence_empty_test",
+    )
+
+    verify_with_ort_with_inputs(model, [], target=target, dev=dev)
+
+
 def test_exporting_node_renamed_model():
     """test exproting model when export_node_renamed_model is set"""