ONNX broken for identity function

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System Information

OpenCV python version: 4.10.0.82 PyTorch version: 2.0.0+cu117 Operating System / Platform: Ubuntu 22.04 Python version: 3.10.6

Detailed description

I have been having some issues with ONNX files lately. Decided to check that the simplest function of them all works: the identity ...

Here is a script that shows for what input shapes ONNX is broken (0, 1 and 3 dimensional inputs). I redirect stdout when exporting the ONNX file to make the output more readable.

I suspect that this is the underlying error to https://github.com/opencv/opencv/issues/25762

Steps to reproduce

import torch
import cv2 as cv
from contextlib import redirect_stdout

class Identity(torch.nn.Module):

    def __init__(self):
        super().__init__()

    def forward(self, X):
        return X

model = Identity()
shape = (2,3,5,7,11,13,17,19)

for i in range(1 + len(shape)):
    X = torch.zeros(shape[:i])
    with open('/dev/null', 'w') as f:
        with redirect_stdout(f):
            torch.onnx.export(model, X, '/tmp/model.onnx', output_names=['Y'])

    Y = model(X)
    ok = X.shape == Y.shape
    print('OK   ' if ok else 'ERROR', 'PyTorch', tuple(X.shape), '->', tuple(Y.shape))

    net = cv.dnn.readNetFromONNX('/tmp/model.onnx')
    X = X.numpy()
    net.setInput(X)

    Y = net.forward(['Y'])[0]
    ok = X.shape == Y.shape
    print('OK   ' if ok else 'ERROR', 'OpenCV ', X.shape, '->', Y.shape)

which gives me the following output (format: Dimension, OK/ERROR, PyTorch/OpenCV, SourceShape, TargetShape)

0 OK    PyTorch () -> ()
0 ERROR OpenCV  () -> (1, 1)
1 OK    PyTorch (2,) -> (2,)
1 ERROR OpenCV  (2,) -> (2, 1)
2 OK    PyTorch (2, 3) -> (2, 3)
2 OK    OpenCV  (2, 3) -> (2, 3)
3 OK    PyTorch (2, 3, 5) -> (2, 3, 5)
3 ERROR OpenCV  (2, 3, 5) -> (2, 3)
4 OK    PyTorch (2, 3, 5, 7) -> (2, 3, 5, 7)
4 OK    OpenCV  (2, 3, 5, 7) -> (2, 3, 5, 7)
5 OK    PyTorch (2, 3, 5, 7, 11) -> (2, 3, 5, 7, 11)
5 OK    OpenCV  (2, 3, 5, 7, 11) -> (2, 3, 5, 7, 11)
6 OK    PyTorch (2, 3, 5, 7, 11, 13) -> (2, 3, 5, 7, 11, 13)
6 OK    OpenCV  (2, 3, 5, 7, 11, 13) -> (2, 3, 5, 7, 11, 13)
7 OK    PyTorch (2, 3, 5, 7, 11, 13, 17) -> (2, 3, 5, 7, 11, 13, 17)
7 OK    OpenCV  (2, 3, 5, 7, 11, 13, 17) -> (2, 3, 5, 7, 11, 13, 17)
8 OK    PyTorch (2, 3, 5, 7, 11, 13, 17, 19) -> (2, 3, 5, 7, 11, 13, 17, 19)
8 OK    OpenCV  (2, 3, 5, 7, 11, 13, 17, 19) -> (2, 3, 5, 7, 11, 13, 17, 19)

As you can see, it's broken for 0, 1 and 3 dimensional inputs.

Issue submission checklist

• [X] I report the issue, it's not a question
• [X] I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• [X] I updated to the latest OpenCV version and the issue is still there
• [X] There is reproducer code and related data files (videos, images, onnx, etc)