[Integration] Added execute_custom_node.py, at the moment without functionality

src/finn/core/execute_custom_node.py

+#import onnx.helper as helper
+
+#import finn.core.MultiThreshold
+
+def execute_custom_node(node, context, graph)
+    """Call custom implementation to execute a single custom node. Input/output provided via context."""
+    node_inputs = list(filter(lambda x: x.name in node.input, graph.input))
+    print(node_inputs)