feat: add marks for Sum and SumSq

subtrees/dagflow/dagflow/lib/Sum.py

@@ -13,6 +13,7 @@ from ..typefunctions import (
 class Sum(FunctionNode):
     """Sum of all the inputs together"""
 
+    _mark = 'Σ'
     def __init__(self, *args, **kwargs):
         kwargs.setdefault(
             "missing_input_handler", MissingInputAddOne(output_fmt="result")

subtrees/dagflow/dagflow/lib/SumSq.py

@@ -14,6 +14,7 @@ class SumSq(FunctionNode):
     """Sum of the squared of all the inputs"""
 
     _buffer: ndarray
+    _mark = 'Σ()²'
     def __init__(self, *args, **kwargs):
         kwargs.setdefault(
             "missing_input_handler", MissingInputAddOne(output_fmt="result")