Loop op with maximum iterations should match the onnx spec

[hwangdeyu]

So far, Loop op with maximum iterations will set to maxint in tf2onnx conversion.

But in Loop ONNX spec https://github.com/onnx/onnx/blob/main/docs/Operators.md#inputs-2
M with empty string to skip matchs the ONNX better.