[ONNX] Move all torch.onnx.export related tests to test/onnx (#87292)
Moving torch.onnx.export related tests to test/onnx integrates ONNX tests to the same CI machine, so the testing environment can be better managed.
Fixes https://github.com/pytorch/pytorch/issues/87320
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87292
Approved by: https://github.com/thiagocrepaldi, https://github.com/BowenBao, https://github.com/kit1980
