[ONNX] Fix assert in cat (#94870)
The assert statement blocks tensors with unknown ranks. This change unblocks those cases. Needed for https://github.com/pytorch/vision/pull/7056
Verified against https://github.com/pytorch/vision/pull/7056
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94870
Approved by: https://github.com/BowenBao
