[CustomOp] add helper function to get custom op for onnx node

src/finn/custom_op/registry.py

 # make sure new CustomOp subclasses are imported here so that they get
 # registered and plug in correctly into the infrastructure
 from finn.custom_op.fpgadataflow.convolutioninputgenerator import (
-    ConvolutionInputGenerator
+    ConvolutionInputGenerator,
 )
 from finn.custom_op.fpgadataflow.streamingfclayer_batch import StreamingFCLayer_Batch
 from finn.custom_op.fpgadataflow.streamingmaxpool_batch import StreamingMaxPool_Batch
@@ -18,3 +18,15 @@ custom_op["StreamingMaxPool_Batch"] = StreamingMaxPool_Batch
 custom_op["StreamingFCLayer_Batch"] = StreamingFCLayer_Batch
 custom_op["ConvolutionInputGenerator"] = ConvolutionInputGenerator
 custom_op["TLastMarker"] = TLastMarker
+
+
+def getCustomOp(node):
+    "Return a FINN CustomOp instance for the given ONNX node, if it exists."
+    op_type = node.op_type
+    try:
+        # lookup op_type in registry of CustomOps
+        inst = custom_op[op_type](node)
+        return inst
+    except KeyError:
+        # exception if op_type is not supported
+        raise Exception("Custom op_type %s is currently not supported." % op_type)