sometime slice need -1 in starts or ends, onnx doc actually allow https://github.com/onnx/onnx/blob/master/docs/Changelog.md#slice-1

starts and ends attributes to specify the start and end dimension for each axis in the list of axes, it uses this information to slice the input data tensor. If a negative value is passed for any of the start or end indices, it represent number of elements before the end of that dimension.

So I think this is a bug. This is blocking a real model conversion, could anybody familiar with this take a look? Thanks a lot.