[Exec] use new custom op infra for exec'ing custom ops

src/finn/core/execute_custom_node.py

 # import onnx.helper as helper
 
-import finn.core.multithreshold as multiThresh
-from finn.core.utils import get_by_name
+import finn.custom_op.registry as registry
 
 
 def execute_custom_node(node, context, graph):
     """Call custom implementation to execute a single custom node.
     Input/output provided via context."""
-
-    if node.op_type == "MultiThreshold":
-        # save inputs
-        v = context[node.input[0]]
-        thresholds = context[node.input[1]]
-        # retrieve attributes if output scaling is used
-        try:
-            out_scale = get_by_name(node.attribute, "out_scale").f
-        except AttributeError:
-            out_scale = None
-        try:
-            out_bias = get_by_name(node.attribute, "out_bias").f
-        except AttributeError:
-            out_bias = None
-        # calculate output
-        output = multiThresh.execute(v, thresholds, out_scale, out_bias)
-        # setting context according to output
-        context[node.output[0]] = output
-
-    else:
+    op_type = node.op_type
+    try:
+        # lookup op_type in registry of CustomOps
+        inst = registry.custom_op[op_type]()
+        inst.execute_node(node, context, graph)
+    except KeyError:
         # exception if op_type is not supported
-        raise Exception("This custom node is currently not supported.")
+        raise Exception("Custom op_type %s is currently not supported." % op_type)