[Test] Added not runable test for model with mixed nodes

tests/test_mixed_onnx_exec.py

+import numpy as np
+from onnx import TensorProto, helper
+
+
+def test_execute_mixed_model():
+
+    out0 = helper.make_tensor_value_info("out", TensorProto.FLOAT, [6, 3, 2, 2])
+
+    graph_def = helper.make_graph(
+        nodes=[
+            helper.make_node(
+                "MultiThreshold", ["v", "thresholds"], ["out"], domain="finn"
+            ),
+            helper.make_node("Relu", ["out0"], ["out1"]),
+        ],
+        name="test-model",
+        inputs=[
+            helper.make_tensor_value_info("v", TensorProto.FLOAT, [6, 3, 2, 2]),
+            helper.make_tensor_value_info("thresholds", TensorProto.FLOAT, [3, 7]),
+        ],
+        outputs=[helper.make_tensor_value_info("out", TensorProto.FLOAT, [6, 3, 2, 2])],
+    )