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

[iksnagreb]

Sometimes you want to seek upstream until you either find a particular node or terminate at an input or initializer. This is almost handled by find_upstream, except for it returning the empty list in case the search terminates. This is reasonable default behavior. By adding a flag, handling the second use case is almost free. The option is disabled by default to not change the existing behavior.

By the way: Is there a particular reason to return the empty list if there is no producer and to return None if there is a producer without inputs? It is probably of interest to distinguish these tow cases, but was this a deliberate choice utilized somewhere? This behavior is not changed by this PR.

[iksnagreb]

Hm, some tests fail due to ValueError: Could not infer attribute explicit_paddings type from empty iterator somewhere within onnx. Nothing I have touched, seems to be due to a recent onnx update while the onnx version here is not pinned exactly, it uses the >= specifier.

[maltanar]

Hi Christoph,
The ValueError: Could not infer attribute explicit_paddings type from empty iterator problems were due to an unrelated package which is now fixed. Adding changes from latest main to re-trigger the CI now.
Regarding the None vs [] return behavior, I believe this is due to some legacy code in FINN that is using one variant or another - @auphelia might have a better idea here. I'm not aware if this is being used to distinguish something useful.

[maltanar]

CI runs now looking good. Since the new parameter defaults to the old behavior when not specified I think this should be a safe inclusion into the codebase. Thanks for the PR!