Fix IBP support for BoundSlice

[hx507]

Hi! It seems like the current IBP implementation will yield error if the model contains Slice operations.

It seems simple to fix that. Please educate me if the fix looks wrong.

[huanzhang12]

@hx507 Thank you so much for your contribution!
@shizhouxing Can you take a look at the changes and merge this into our next release?

[shizhouxing]

Thanks @hx507!

@huanzhang12 It looks great. Shall I create a develop branch and merge this commit to develop? Later we can add our commits to develop and then merge them to master together? Or do we directly merge it to master? I think we need to ensure commits from the community are not squashed in future releases.

[huanzhang12]

@shizhouxing Thanks for merging it! Yes this commit can stay in develop for now, and we will update the master branch in our next release (which will be in late October).