Add option to find_upstream to keep nodes visited even if not found

src/qonnx/core/modelwrapper.py

@@ -346,16 +346,19 @@ def find_producer(self, tensor_name):
                 return x
         return None
 
-    def find_upstream(self, tensor_name, finder_fxn):
+    def find_upstream(self, tensor_name, finder_fxn, keep_if_not_found=False):
         """Follow the producer chain upstream, calling finder_fxn on each upstream
         node until it returns True or there are no nodes left. Returns the list
-        of nodes visited, or None if finder_fxn did not return True."""
+        of nodes visited, or None if finder_fxn did not return True. If
+        keep_if_not_found is specified, returns the list of nodes visited, even
+        if finder_fxn never returned True, i.e., if the search terminated at an
+        input or initializer."""
         visit_list = []
         current_tensor = tensor_name
         while True:
             current_producer = self.find_producer(current_tensor)
             if current_producer is None:
-                return []
+                return visit_list if keep_if_not_found else []
             else:
                 found = finder_fxn(current_producer)
                 visit_list.append(current_producer)
@@ -364,7 +367,7 @@ def find_upstream(self, tensor_name, finder_fxn):
                 elif len(current_producer.input) > 0:
                     current_tensor = current_producer.input[0]
                 else:
-                    return None
+                    return visit_list if keep_if_not_found else None
 
     def find_consumer(self, tensor_name):
         """Finds and returns the node that consumes the tensor with given name.