Add cleanup transformation sorting inputs of commutative operations

[iksnagreb]

Adds the cleanup transformation requested in #84

Closes #84

[maltanar]

Hi Christoph,

Thanks for the PR. I agree that this would be a useful cleanup transformation, but one remark after a brief review: since this gets called as part of the ModelWrapper.cleanup() which essentially gets called after every single transformation, and since the ONNX export behavior we observe from e.g. PyTorch has changed over the course of versions, we should try to add some extra checks for future-proofing here. Could you provide a unit test that exercises this transformation? Might be easiest to take one of the existing ONNX testcases and artificially manipulate the input list as part of the testcase such that the initialized input appears first.

[iksnagreb]

Could you provide a unit test that exercises this transformation?

Has been added now, testing this transformation on the Sum operator and some input/initializer combinations.

[maltanar]

LGTM, ready to merge. Thanks @iksnagreb !